Fix stack overflow (#807)

Fix stack overflow in HandleNode by explicitly limiting the depth of recursion.

src/depthguard.cpp

@@ -0,0 +1,10 @@
+#include "yaml-cpp/depthguard.h"
+
+namespace YAML {
+
+DeepRecursion::DeepRecursion(int depth, const Mark& mark_, const std::string& msg_)
+    : ParserException(mark_, msg_),
+      m_depth(depth) {
+}
+
+} // namespace YAML

src/singledocparser.cpp

@@ -7,6 +7,7 @@
 #include "singledocparser.h"
 #include "tag.h"
 #include "token.h"
+#include "yaml-cpp/depthguard.h"
 #include "yaml-cpp/emitterstyle.h"
 #include "yaml-cpp/eventhandler.h"
 #include "yaml-cpp/exceptions.h"  // IWYU pragma: keep
@@ -47,6 +48,8 @@ void SingleDocParser::HandleDocument(EventHandler& eventHandler) {
 }
 
 void SingleDocParser::HandleNode(EventHandler& eventHandler) {
+  DepthGuard<2000> depthguard(depth, m_scanner.mark(), ErrorMsg::BAD_FILE);
+
   // an empty node *is* a possibility
   if (m_scanner.empty()) {
     eventHandler.OnNull(m_scanner.mark(), NullAnchor);

src/singledocparser.h

@@ -15,6 +15,7 @@
 
 namespace YAML {
 class CollectionStack;
+template <int> class DepthGuard; // depthguard.h
 class EventHandler;
 class Node;
 class Scanner;
@@ -55,6 +56,7 @@ class SingleDocParser {
   anchor_t LookupAnchor(const Mark& mark, const std::string& name) const;
 
  private:
+  int depth = 0;
   Scanner& m_scanner;
   const Directives& m_directives;
   std::unique_ptr<CollectionStack> m_pCollectionStack;