Some improvements to fuzzers

[nmoinvaz]

• Use different fuzzer bits in example_dict_fuzzer for more input value combinations.
  • This is similar to changes made in Improvements to minigzip fuzzer coverage #929 but for example_dict_fuzzer instead of minigzip_fuzzer.
• Fixed gz mode for compression level not using ascii numeric value.
  • This is a bug in minigzip_fuzzer writing the gz mode. Previously minigzip_fuzzer wouldn't have been testing all compression levels.
• Fixed trying to uncompress after compressing in gzip direct mode which is not supported by gz functions.
  • Changes made by Improvements to minigzip fuzzer coverage #929 caused this issue. This should fix OSS-Fuzz issue https://oss-fuzz.com/testcase-detail/6194422837542912.

[codecov]

Codecov Report

Merging #977 (93eed04) into develop (c918062) will decrease coverage by 1.67%.
The diff coverage is 71.42%.

@@             Coverage Diff             @@
##           develop     #977      +/-   ##
===========================================
- Coverage    77.46%   75.79%   -1.68%     
===========================================
  Files           74       74              
  Lines         8313     8084     -229     
  Branches      1374     1344      -30     
===========================================
- Hits          6440     6127     -313     
- Misses        1339     1426      +87     
+ Partials       534      531       -3     

Flag	Coverage Δ
macos_clang	68.54% <71.42%> (-0.01%)	⬇️
macos_gcc	67.47% <83.33%> (+0.01%)	⬆️
ubuntu_clang	?
ubuntu_clang_debug	?
ubuntu_clang_inflate_allow_invalid_dist	?
ubuntu_clang_inflate_strict	?
ubuntu_clang_mmap	?
ubuntu_clang_msan	?
ubuntu_gcc	68.72% <83.33%> (-5.00%)	⬇️
ubuntu_gcc_aarch64	69.00% <83.33%> (-5.25%)	⬇️
ubuntu_gcc_aarch64_compat_no_opt	67.31% <83.33%> (-5.66%)	⬇️
ubuntu_gcc_aarch64_no_acle	67.91% <83.33%> (-5.38%)	⬇️
ubuntu_gcc_aarch64_no_neon	68.31% <83.33%> (-5.43%)	⬇️
ubuntu_gcc_armhf	68.99% <83.33%> (-5.26%)	⬇️
ubuntu_gcc_armhf_compat_no_opt	67.30% <83.33%> (-5.68%)	⬇️
ubuntu_gcc_armhf_no_acle	69.15% <83.33%> (-5.29%)	⬇️
ubuntu_gcc_armhf_no_neon	69.48% <83.33%> (-5.35%)	⬇️
ubuntu_gcc_armsf	69.00% <83.33%> (-5.26%)	⬇️
ubuntu_gcc_armsf_compat_no_opt	67.30% <83.33%> (-5.68%)	⬇️
ubuntu_gcc_compat_no_opt	68.69% <83.33%> (-5.49%)	⬇️
ubuntu_gcc_mingw_i686	0.00% <0.00%> (ø)
ubuntu_gcc_mingw_x86_64	0.00% <0.00%> (ø)
ubuntu_gcc_no_avx2	69.07% <83.33%> (-5.12%)	⬇️
ubuntu_gcc_no_pclmulqdq	66.90% <83.33%> (-5.22%)	⬇️
ubuntu_gcc_no_sse2	68.13% <83.33%> (-5.05%)	⬇️
ubuntu_gcc_no_sse4	66.86% <83.33%> (-5.23%)	⬇️
ubuntu_gcc_o3	68.45% <83.33%> (-5.15%)	⬇️
ubuntu_gcc_osb	70.99% <83.33%> (-5.53%)	⬇️
ubuntu_gcc_ppc	69.18% <83.33%> (-5.41%)	⬇️
ubuntu_gcc_ppc64	69.97% <83.33%> (-5.39%)	⬇️
ubuntu_gcc_ppc64le	68.67% <83.33%> (-5.42%)	⬇️
ubuntu_gcc_s390x	67.92% <83.33%> (-5.15%)	⬇️
ubuntu_gcc_sparc64	70.37% <83.33%> (-5.44%)	⬇️
win64_gcc	70.40% <83.33%> (+0.01%)	⬆️
win64_gcc_compat_no_opt	73.87% <83.33%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
test/fuzz/minigzip_fuzzer.c	50.00% <50.00%> (-2.39%)	⬇️
test/fuzz/example_dict_fuzzer.c	81.57% <100.00%> (-0.24%)	⬇️
zutil.c	37.50% <0.00%> (-44.65%)	⬇️
deflate.h	8.33% <0.00%> (-41.67%)	⬇️
trees.c	65.07% <0.00%> (-31.15%)	⬇️
trees_emit.h	67.53% <0.00%> (-27.53%)	⬇️
zutil_p.h	40.00% <0.00%> (-26.67%)	⬇️
arch/x86/compare258_avx.c	96.42% <0.00%> (-3.58%)	⬇️
test/fuzz/example_small_fuzzer.c	86.44% <0.00%> (-2.09%)	⬇️
arch/x86/adler32_avx.c	98.33% <0.00%> (-1.67%)	⬇️
... and 31 more

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[Dead2]

I would like to see coverage diff for this one.
But it seems all the Clang-based coverage has stopped working..

Unexpected version: *804.
Invalid .gcno File!

[nmoinvaz]

See #981. I don't think this PR will change code coverage very much if any at all. All the compression levels are already tested by our other tests. And during testing we only test these fuzzers with a limited number of files.