reject invalid symbol table entry size in FindSymbol

[nabhan06]

Unchecked symbol-table entry size in FindSymbol
symtab->sh_entsize is read verbatim from the object file and used as the divisor for num_symbols and as the per-entry read stride. A crafted ELF whose SHT_SYMTAB/SHT_DYNSYM header carries an entry size of zero divides by zero here; any other value that differs from sizeof(ElfW(Sym)) slides the fixed-size symbol reads off the real entries. Added the same exact-size check the section-header paths already apply to e_shentsize, so a malformed table is skipped instead of trusted.

[mkruskal-google]

Can you add a test case that demonstrates this bug (and fails without the fix)?

[mkruskal-google]

requesting changes*

[nabhan06]

Added one in symbolize_test.cc (InvalidSymtabEntSizeDoesNotCrash). It builds a minimal ELF whose SHT_SYMTAB header has sh_entsize == 0, maps an executable page, and points the symbolizer at that file via a mapping hint so FindSymbol actually runs over it. Without the guard the num_symbols = sh_size / sh_entsize divide-by-zero SIGFPEs during symbolization; with it the malformed table is skipped and the address just fails to resolve. Reused the same RegisterFileMappingHint approach as SymbolizeWithMultipleMaps, and it's guarded to the ELF symbolizer.