Relax an assert when there's a type mismatch in forward references

Summary:
We don't seem to need to assert here, since this function's callers expect
to get a nullptr on error. This way we don't assert on user input.

Bug found with AFL fuzz.

Reviewers: rafael

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9308

llvm-svn: 236027