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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236027 91177308-0d34-0410-b5e6-96231b3b80d8