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