[Object/ELF] - Check index argument in getSymbol().

Without this check LLD crashes when SHT_GROUP section has invalid symbol index
because of next code:

template <class ELFT>
StringRef elf::ObjectFile<ELFT>::getShtGroupSignature(const Elf_Shdr &Sec) {
..
  const Elf_Sym *Sym = Obj.getSymbol(Symtab, Sec.sh_info);
..
}
If sh_info is too large, &Symbols[Index] just asserts.

No testcases provided because llvm-objdump/llvm-readelf does 
not use getSymbol() function.

I`ll commit testcase for LLD separatelly.

Differential revision: https://reviews.llvm.org/D25516

llvm-svn: 284702