Add missing clang-format null pointer check..

.. and a test that triggers it in valid albeit questionable code.

llvm-svn: 175554
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
index 3a1e9dc..05e5d37 100644
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -937,6 +937,8 @@
                getLLVMStyleWithColumns(20));
 
   verifyFormat("#define A template <typename T>");
+  verifyFormat("#define STR(x) #x\n"
+               "f(STR(this_is_a_string_literal{));");
 }
 
 TEST_F(FormatTest, IndentPreprocessorDirectivesAtZero) {