[llvm/Object] - Fix the error message reported for a broken SHT_SYMTAB_SHNDX section.

SHT_SYMTAB_SHNDX should have the same number of entries as the symbol table
associated (https://www.sco.com/developers/gabi/latest/ch4.sheader.html)

We currently can report the following message:
"SHT_SYMTAB_SHNDX section has sh_size (24) which is not equal to the number of symbols (2)"

It is just broken. This patch refines/fixes it.

Differential revision: https://reviews.llvm.org/D69305