[ELF] - Do not segfault when doing logical and/or operations on symbols that...

[ELF] - Do not segfault when doing logical and/or operations on symbols that have no output sections.

Previously we would crash on samples from testcase,
because were trying to access zero pointer to output section.

Differential revision: https://reviews.llvm.org/D36145

llvm-svn: 311311