Fix infinite-loop in unwrapped line parser.

Discovered when accidentally formatting a python file :-).

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@177527 91177308-0d34-0410-b5e6-96231b3b80d8
Remove assertion that can be triggered on bad input.

clang-format can't do anything useful, so it should leave the remainder
of the line unchanged, but it should not assert/segfault.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@177530 91177308-0d34-0410-b5e6-96231b3b80d8