When a relocation to an undefined symbol is an R_X86_64_PC32, an input

When a relocation to an undefined symbol is an R_X86_64_PC32, an input
section will not have an input file. Don't crash under those circumstances.

Neither clang nor llvm-mc generates R_X86_64_PC32 relocations due to
https://reviews.llvm.org/D43383, which makes it hard to write a test case.
However, gcc does generate such relocations. I want to get a fix in now,
but will figure out a way to actually exercise this code path as soon
as I can.

llvm-svn: 341408