Fix clang-format emacs integration in last line.

Emacs seems to have a line that is just past the last character of the
buffers content. This needs to be handled specially so that clang-format
is not called with an invalid -offset.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181299 91177308-0d34-0410-b5e6-96231b3b80d8