mmappend.c: pre-check header trailer magic

[keentux]

• Avoid potential ASAN:SIGSEGV invalid memory access by pre-check the header trailer magic. Somehow, the pointer could overflow in the buffer especially for 32bits architecture where the value become higher of the maximal value of an uint32_t. Checking the value of the pointer before trying to read the zzip_file_header struct will avoid a potential "invalid memory access".
• CVE-2020-18770 from #69