[ELF] - Do not crash on invalid symbol index.

Relative to PR30540.

If .symtab has invalid type in elf, no bodies are created and any relocation
that tries to access them will fail.
The same can happen if symbol index is just incorrect.

This was revealed by "id_000005,sig_11,src_000000,op_flip2,pos_420"

Differential revision: https://reviews.llvm.org/D25025

llvm-svn: 283201