Fix clang-format segfault.

When annotating "lines" starting with ":", clang-format would segfault.
This could actually happen in valid code, e.g.
  #define A :

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177283 91177308-0d34-0410-b5e6-96231b3b80d8