Fix integer overflow during parsing

[sanmai-NL]

cargo +nightly-2017-10-05 fuzz run utf8_parse_response -- -only_ascii=1

      Fresh arbitrary v0.1.0
       Fresh cc v1.0.3                                                                                                                                                    
       Fresh libc v0.2.33                                                                                                                                                 
       Fresh memchr v1.0.2                                                                                                                                                
       Fresh nom v3.2.1                                                                                                                                                   
       Fresh libfuzzer-sys v0.1.0 (https://github.com/rust-fuzz/libfuzzer-sys.git#737524f7)                                                                               
       Fresh imap-proto v0.1.0 (file:///home/sanmai/devel/github.com/djc/imap-proto)                                                                                      
   Compiling imap-proto-fuzz v0.0.1 (file:///home/sanmai/devel/github.com/djc/imap-proto/fuzz)                                                                            
     Running `rustc --crate-name utf8_parse_response fuzz/fuzz_targets/utf8_parse_response.rs --crate-type bin --emit=dep-info,link -C debuginfo=2 -C metadata=a03a0cad977a1d2a -C extra-filename=-a03a0cad977a1d2a --out-dir /home/sanmai/devel/github.com/djc/imap-proto/fuzz/target/x86_64-unknown-linux-gnu/debug/deps --target x86_64-unknown-linux-gnu -L dependency=/home/sanmai/devel/github.com/djc/imap-proto/fuzz/target/x86_64-unknown-linux-gnu/debug/deps -L dependency=/home/sanmai/devel/github.com/djc/imap-proto/fuzz/target/debug/deps --extern imap_proto=/home/sanmai/devel/github.com/djc/imap-proto/fuzz/target/x86_64-unknown-linux-gnu/debug/deps/libimap_proto-1f870aaee1b4c109.rlib --extern libfuzzer_sys=/home/sanmai/devel/github.com/djc/imap-proto/fuzz/target/x86_64-unknown-linux-gnu/debug/deps/liblibfuzzer_sys-76eea54a6581bf0a.rlib --cfg fuzzing -Cpasses=sancov -Cllvm-args=-sanitizer-coverage-level=3 -Zsanitizer=address -Cpanic=abort -L native=/home/sanmai/devel/github.com/djc/imap-proto/fuzz/target/x86_64-unknown-linux-gnu/debug/build/libfuzzer-sys-dfeac7893557483c/out`                                                                                                         
    Finished dev [unoptimized + debuginfo] target(s) in 0.82 secs                                                                                                         
       Fresh cc v1.0.3                                                                                                                                                    
       Fresh libc v0.2.33                                                                                                                                                 
       Fresh arbitrary v0.1.0                                                                                                                                             
       Fresh memchr v1.0.2                                                                                                                                                
       Fresh nom v3.2.1
       Fresh libfuzzer-sys v0.1.0 (https://github.com/rust-fuzz/libfuzzer-sys.git#737524f7)
       Fresh imap-proto v0.1.0 (file:///home/sanmai/devel/github.com/djc/imap-proto)
       Fresh imap-proto-fuzz v0.0.1 (file:///home/sanmai/devel/github.com/djc/imap-proto/fuzz)
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `fuzz/target/x86_64-unknown-linux-gnu/debug/utf8_parse_response -artifact_prefix=/home/sanmai/devel/github.com/djc/imap-proto/fuzz/artifacts/utf8_parse_response/ -only_ascii=1 /home/sanmai/devel/github.com/djc/imap-proto/fuzz/corpus/utf8_parse_response`
INFO: Seed: 408015737
INFO: Loaded 0 modules (0 guards): 
Loading corpus dir: /home/sanmai/devel/github.com/djc/imap-proto/fuzz/corpus/utf8_parse_response
INFO: -max_len is not provided, using 64
#0      READ units: 89
#89     INITED cov: 2409 corp: 69/1418b exec/s: 0 rss: 155Mb
thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: ParseIntError { kind: Overflow }', /checkout/src/libcore/result.rs:906:4
note: Run with `RUST_BACKTRACE=1` for a backtrace.
==15504== ERROR: libFuzzer: deadly signal
    #0 0x560481414373 in __sanitizer_print_stack_trace /checkout/src/libcompiler_builtins/compiler-rt/lib/asan/asan_stack.cc:38
    #1 0x5604811519cd in fuzzer::Fuzzer::CrashCallback() /home/sanmai/.cargo/bin/git/checkouts/libfuzzer-sys-e07fde05820d7bc6/737524f/llvm/lib/Fuzzer/FuzzerLoop.cpp:280
    #2 0x560481151917 in fuzzer::Fuzzer::StaticCrashSignalCallback() /home/sanmai/.cargo/bin/git/checkouts/libfuzzer-sys-e07fde05820d7bc6/737524f/llvm/lib/Fuzzer/FuzzerLoop.cpp:264
    #3 0x56048117b487 in fuzzer::CrashHandler(int, siginfo_t*, void*) /home/sanmai/.cargo/bin/git/checkouts/libfuzzer-sys-e07fde05820d7bc6/737524f/llvm/lib/Fuzzer/FuzzerUtilPosix.cpp:37
    #4 0x7f5dd1393d9f  (/usr/lib/libpthread.so.0+0x11d9f)
    #5 0x7f5dd0de789f in __GI_raise (/usr/lib/libc.so.6+0x3489f)
    #6 0x7f5dd0de8f08 in __GI_abort (/usr/lib/libc.so.6+0x35f08)
    #7 0x5604813459f8 in panic_abort::__rust_start_panic::abort /checkout/src/libpanic_abort/lib.rs:59
    #8 0x5604813459f8 in __rust_start_panic /checkout/src/libpanic_abort/lib.rs:54

NOTE: libFuzzer has rudimentary signal handlers.
      Combine libFuzzer with AddressSanitizer or similar for better crash reports.
SUMMARY: libFuzzer: deadly signal
MS: 1 InsertRepeatedBytes-; base unit: f74b576b1b65321c5909433dc2e3f79220af8725
0x2a,0x20,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x43,
* 2222222222222222222222222222222222222222222C
artifact_prefix='/home/sanmai/devel/github.com/djc/imap-proto/fuzz/artifacts/utf8_parse_response/'; Test unit written to /home/sanmai/devel/github.com/djc/imap-proto/fuzz/artifacts/utf8_parse_response/crash-c91935afc8db1ba7c29aa83a549bc90ffac1d312
Base64: KiAyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyQw==
==15504==LeakSanitizer has encountered a fatal error.
==15504==HINT: For debugging, try setting environment variable LSAN_OPTIONS=verbosity=1:log_threads=1
==15504==HINT: LeakSanitizer does not work under ptrace (strace, gdb, etc)
MS: 1 InsertRepeatedBytes-; base unit: f74b576b1b65321c5909433dc2e3f79220af8725
0x2a,0x20,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x32,0x43,
* 2222222222222222222222222222222222222222222C
artifact_prefix='/home/sanmai/devel/github.com/djc/imap-proto/fuzz/artifacts/utf8_parse_response/'; Test unit written to /home/sanmai/devel/github.com/djc/imap-proto/fuzz/artifacts/utf8_parse_response/crash-c91935afc8db1ba7c29aa83a549bc90ffac1d312
Base64: KiAyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyQw==

test unit

* 2222222222222222222222222222222222222222222C