Don't use `DebugLoc` accessors if we're pointing at null, which will be
a problem after a WIP patch to make the `DIDescriptor` accessors more
strict. Caught by Frontend/profile-sample-use-loc-tracking.c (in
clang).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232792 91177308-0d34-0410-b5e6-96231b3b80d8