Add a test for vcall on a null ptr.

Summary:
Turns out that in the case of -fsanitize=null and a virtual call,
the type check was generated *after* reading from vtable, which
causes a non-interpretable segfault. The check has been moved up
in https://reviews.llvm.org/D26559 and this CL adds a test for this case.

Reviewers: pcc

Subscribers: cfe-commits, kubabrecka

Differential Revision: https://reviews.llvm.org/D26560

llvm-svn: 287578