[clang-tidy] Fix a crash in fuchsia-multiple-inheritance

Summary: See the test case for a repro.

Reviewers: juliehockett, ioeric, hokein, aaron.ballman

Reviewed By: hokein

Subscribers: lebedev.ri, xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D49862

llvm-svn: 338124