fix: change abort when no parsing progress is made

Previously, no parsing progress (detecting infinite loops) was detected by an empty Node.
This falsely triggers an abort, if an empty YAML document is being parsed.

Instead, we detect if progress in the parsing stream is made, by
comparing token positions. As long as new tokens are being
parsed (detected by position change), we assume we are not in an
infinite loop.

src/parse.cpp

@@ -53,7 +53,7 @@ std::vector<Node> LoadAll(std::istream& input) {
   Parser parser(input);
   while (true) {
     NodeBuilder builder;
-    if (!parser.HandleNextDocument(builder) || builder.Root().IsNull()) {
+    if (!parser.HandleNextDocument(builder)) {
       break;
     }
     docs.push_back(builder.Root());

src/parser.cpp

@@ -33,11 +33,26 @@ bool Parser::HandleNextDocument(EventHandler& eventHandler) {
     return false;
   }
 
+  auto oldPos = m_pScanner->peek().mark.pos;
+
   SingleDocParser sdp(*m_pScanner, *m_pDirectives);
   sdp.HandleDocument(eventHandler);
+
+  // checks if progress was made
+  // 1. if scanner has no more tokens, progress was made
+  if (m_pScanner->empty()) {
     return true;
   }
 
+  // 2. if token position has changed, progress was made
+  auto newPos = m_pScanner->peek().mark.pos;
+  if (newPos != oldPos) {
+    return true;
+  }
+  // No progress was made, no further processing
+  return false;
+}
+
 void Parser::ParseDirectives() {
   bool readDirective = false;