[clang-format] fix crash involving invalid preprocessor line

[krasimir]

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.

[gribozavr]

unittests/Format/FormatTest.cpp

3090

// #elif directive without a condition. -- to follow the existing comment format in other tests.

[krasimir]

• Apply review comments

[krasimir]

unittests/Format/FormatTest.cpp

3090

done

lib/Format/TokenAnnotator.cpp

while (CurrentToken) {
                             Keywords.kw___has_include_next))
         parseHasInclude();
     }
     return Type;
   }
 
 public:
   LineType parseLine() {
+    if (!CurrentToken)
+      return LT_Invalid;
     NonTemplateLess.clear();
     if (CurrentToken->is(tok::hash))
       return parsePreprocessorDirective();
 
     // Directly allow to 'import <string-literal>' to support protocol buffer
     // definitions (github.com/google/protobuf) or missing "#" (either way we
     // should not break the line).
     IdentifierInfo *Info = CurrentToken->Tok.getIdentifierInfo();

unittests/Format/FormatTest.cpp

TEST_F(FormatTest, IndentPreprocessorDirectives) {
   verifyFormat("#ifndef FOO\n"
                "#define FOO\n"
                "#if 1\n"
                "int i;\n"
                "#  define A 0\n"
                "#endif\n"
                "#endif\n",
                Style);
+  // Don't crash if there is nothing following #elif.
+  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
   // previous code line yet, so we can't detect it.
   EXPECT_EQ("#ifndef NOT_GUARD\n"
             "code();\n"
             "#define NOT_GUARD\n"
             "code();\n"