[clang-tidy] Do not dereference a null BaseType

Check BaseType before dereference.
Simplified test case is derived from Android Open Source code.

Differential Revision: https://reviews.llvm.org/D33430

Change-Id: I06e5c33489f48de29e7873718223c444561a940c
git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@303645 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed