Fix potential use of uninitialized variable.

[dkasak]

It appears in some cases control might jump to the end of fuzz_one via the abandon_entry label, where it gets freed, prior to position_map initialization. Setting it to NULL turns this into a harmless free(NULL). Might manifest as a double free in some cases, probably due to reuse of old stack space in a subsequent call.

I catched this by compiling afl with ASAN, which gave the following report when it crashed:

==8432==ERROR: AddressSanitizer: attempting double-free on 0x9951d440 in thread T0:
    #0 0x56705605 in __interceptor_free.localalias.0 (/home/dkasak/code/projects/afl-rb/afl-fuzz+0xe6605)
    #1 0x5676f5d5 in fuzz_one /home/dkasak/code/projects/afl-rb/afl-fuzz.c:7595:3
    #2 0x5674631b in main /home/dkasak/code/projects/afl-rb/afl-fuzz.c:9068:20
    #3 0xf74157c2 in __libc_start_main (/usr/lib32/libc.so.6+0x187c2)
    #4 0x5663e0a5 in _start (/home/dkasak/code/projects/afl-rb/afl-fuzz+0x1f0a5)

0x9951d440 is located 0 bytes inside of 448-byte region [0x9951d440,0x9951d600)
freed by thread T0 here:
    #0 0x56705605 in __interceptor_free.localalias.0 (/home/dkasak/code/projects/afl-rb/afl-fuzz+0xe6605)
    #1 0x5676f5d5 in fuzz_one /home/dkasak/code/projects/afl-rb/afl-fuzz.c:7595:3
    #2 0x5674631b in main /home/dkasak/code/projects/afl-rb/afl-fuzz.c:9068:20
    #3 0xf74157c2 in __libc_start_main (/usr/lib32/libc.so.6+0x187c2)

previously allocated by thread T0 here:
    #0 0x567057cd in malloc (/home/dkasak/code/projects/afl-rb/afl-fuzz+0xe67cd)
    #1 0x5676dd09 in fuzz_one /home/dkasak/code/projects/afl-rb/afl-fuzz.c:7420:22
    #2 0x5674631b in main /home/dkasak/code/projects/afl-rb/afl-fuzz.c:9068:20
    #3 0xf74157c2 in __libc_start_main (/usr/lib32/libc.so.6+0x187c2)