[ELF] - Fixed crash on invalid input.

I took the input from https://llvm.org/bugs/show_bug.cgi?id=30540, it was
"id_000000,sig_11,src_000000,op_flip1,pos_98"

File contains invalid symbol name offset (too large) and lld just crashes,
patch fixes the issue.

Differential revision: https://reviews.llvm.org/D24970

llvm-svn: 282709