[ELF] - Do not crash on invalid size of dynamic section.

Previously if sh_size of dynamic section was broken,
lld may crash. Or even may not crash if used 32 bits host.
(then value may be truncated to 32 bits when doing pointer arithmetic 
and could be just zero).
Patch fixes the issue.

Differential revision: https://reviews.llvm.org/D25327

llvm-svn: 283533