[clang-tidy] Fix a crash issue on misc-virtual-near-miss check.

Summary:
The crash is caused by triggering a Assertion failed in DeclCXX.h when the
check detects non-defined class return type in a class method declaration.

Reviewers: congliu, alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D16854

llvm-svn: 259668