[LLD][ELF] - Fix crash when using empty --defsym.

We have a crash issue when handling the empty -defsym.

For parsing this option we are using ScriptParser class which is used
generally for reading the linker script. For empty defsym case, we
pass the empty memory buffer and crash in the place removed in https://reviews.llvm.org/rL336436.

But reverting of the above patch would not help here (we would still crash but a bit later). And
even after fixing the crash we would report something like
"lld.exe: error: -defsym:1: unexpected EOF"
It is probably not the appropriate message because mentions EOF.

I think the issue should be handled on a higher level like this patch does.
So we do not want to pass the empty memory buffer first of all I believe.

Differential revision: https://reviews.llvm.org/D50498

llvm-svn: 339412