clang-format: Fix assertion when trying to build a nullptr StringRef.

llvm-svn: 226448
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
index 5f8fba26e..7987e58 100644
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -2600,6 +2600,7 @@
                "    y)), // expected-warning",
                getLLVMStyleWithColumns(28));
   verifyFormat("#d, = };");
+  verifyFormat("#if \"a");
 }
 
 TEST_F(FormatTest, MacrosWithoutTrailingSemicolon) {