[dsymutil] Don't resolve DIE reference to NULL DIE.

This patch prevents dsymutil from resolving a reference to a NULL DIE
when a bogus reference happens to be coincidentally referencing a NULL
DIE. Now this is detected as an invalid reference and a warning is
printed.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=33873

Differential revision: https://reviews.llvm.org/D38078

llvm-svn: 313872