[clang-tidy] Fix an assert failure in 'readability-braces-around-statements' check.

Summary:
The check will trigger a assert failure("CondEndLoc.isValid") when
checking the IfStmt whose condition expression is not parsed.

In this case, we should ignore that.

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D17069

llvm-svn: 260505