[dsymutil] Check AttrInfo.Name validity before using it

Summary:
This upstreams a patch from the osxcross [1] toolchain.
It appears that llvm-dsymutil crashes at this place during GCC
bootstrap. Adding the check here seems reasonable, since it operates
on arbitrary input DWARF, not necessarily generated by the LLVM
toolchain, and it seems the un-mangled name need not necessarily exist.

Patch by Thomas Pöchtrager

[1] https://github.com/tpoechtrager/osxcross

Reviewed By: aprantl
Differential Revision: https://reviews.llvm.org/D39336

llvm-svn: 316678