[ASTMatchers]: fix crash in hasReturnValue

Summary:
The crash was reproduced by the included test case. It was initially
found through a crash of clang-tidy's misc-misplaced-widening-cast
check.

Reviewers: klimek, alexfh

Subscribers: cfe-commits, klimek

Differential Revision: http://reviews.llvm.org/D18991

llvm-svn: 266043