Fix bug 20125 - clang-format segfaults on bad config.

The problem was in unchecked dyn_cast inside of Input::createHNodes.
Patch by Roman Kashitsyn!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215205 91177308-0d34-0410-b5e6-96231b3b80d8