Fix null dereference in yaml::Document::skip

[thomasfinch]

Summary

The attached test case replicates a null dereference crash in yaml::Document::skip(). This was fixed by adding a check and early return in the method.

[hintonda]

LGTM, but I'll wait a few days to land it for you to give @Bigcheese a chance to comment.

llvm/lib/Support/YAMLParser.cpp

Document::Document(Stream &S) : stream(S), Root(nullptr) {
   Token &T = peekNext();
   if (T.Kind == Token::TK_DocumentStart)
     getNext();
 }
 
 bool Document::skip()  {
   if (stream.scanner->failed())
     return false;
-  if (!Root)
-    getRoot();
+  if (!Root && !getRoot())
+    return false;
   Root->skip();
   Token &T = peekNext();
   if (T.Kind == Token::TK_StreamEnd)
     return false;
   if (T.Kind == Token::TK_DocumentEnd) {
     getNext();
     return skip();
   }

llvm/unittests/Support/YAMLParserTest.cpp

TEST(YAMLParser, DifferentNodesIteratorOperatorEquals) {
   auto AnotherBegin = AnotherNode->begin();
   auto AnotherEnd = AnotherNode->end();
 
   EXPECT_FALSE(Begin == AnotherBegin);
   EXPECT_FALSE(Begin == AnotherEnd);
   EXPECT_TRUE(End == AnotherEnd);
 }
 
+TEST(YAMLParser, FlowSequenceTokensOutsideFlowSequence) {
+  auto FlowSequenceStrs = {",", "]", "}"};
+  SourceMgr SM;
+
+  for (auto &Str : FlowSequenceStrs) {
+    yaml::Stream Stream(Str, SM);
+    yaml::Document &Doc = *Stream.begin();
+    EXPECT_FALSE(Doc.skip());
+  }
+}
+
 } // end namespace llvm