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
diff --git a/unittests/Format/FormatTest.cpp b/unittests/Format/FormatTest.cpp
index 4ac9bce..e6ae81d 100644
--- a/unittests/Format/FormatTest.cpp
+++ b/unittests/Format/FormatTest.cpp
@@ -1068,6 +1068,8 @@
 }
 
 TEST_F(FormatTest, MacroDefinitionsWithIncompleteCode) {
+  verifyFormat("#define A :");
+
   // FIXME: Improve formatting of case labels in macros.
   verifyFormat("#define SOMECASES  \\\n"
                "case 1:            \\\n"