[clang-format] fix crash involving invalid preprocessor line

Summary:
This (invalid) fragment is crashing clang-format:
```
#if 1
int x;
#elif
int y;
#endif
```

The reason being that the parser expects a token after `#elif`, and the
subsequent parsing of the next line does not check if `CurrentToken` is null.

Reviewers: gribozavr

Reviewed By: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65940

llvm-svn: 368280

clang/lib/Format/TokenAnnotator.cpp

@@ -1099,6 +1099,8 @@ class AnnotatingParser {
 
 public:
   LineType parseLine() {
+    if (!CurrentToken)
+      return LT_Invalid;
     NonTemplateLess.clear();
     if (CurrentToken->is(tok::hash))
       return parsePreprocessorDirective();

clang/unittests/Format/FormatTest.cpp

@@ -3087,6 +3087,15 @@ TEST_F(FormatTest, IndentPreprocessorDirectives) {
                "#endif\n"
                "#endif\n",
                Style);
+  // Don't crash if there is an #elif directive without a condition.
+  verifyFormat("#if 1\n"
+               "int x;\n"
+               "#elif\n"
+               "int y;\n"
+               "#else\n"
+               "int z;\n"
+               "#endif",
+               Style);
   // FIXME: This doesn't handle the case where there's code between the
   // #ifndef and #define but all other conditions hold. This is because when
   // the #define line is parsed, UnwrappedLineParser::Lines doesn't hold the