[clang] Fix a null pointer dereference.

Summary:
Sometimes expression inside switch statement can be invalid, for
example type might be incomplete. In those cases code were causing a null
pointer dereference. This patch fixes that.

Reviewers: sammccall, ioeric, hokein

Reviewed By: sammccall

Subscribers: arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D53561

llvm-svn: 345029