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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313872 91177308-0d34-0410-b5e6-96231b3b80d8