[Object/ELF] - Avoid possible crash in getExtendedSymbolTableIndex().

When using broken input object found using AFL,
getExtendedSymbolTableIndex() crashed because ShndxTable
was empty as object does not contain SHT_SYMTAB_SHNDX section.

Differential revision: https://reviews.llvm.org/D25189

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@283196 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed