Return fallback if .as(fallback) called on a node that doesn't exist

Since 1025f76df1 it seems that missing
nodes point to a 'zombie' node, where the m_isValid member is false.
This caused .as(const S& fallback) throw an exception if the node was
missing, instead of returning the provided fallback value (as was the
behaviour previous to the above commit)

include/yaml-cpp/node/impl.h

@@ -149,7 +149,7 @@ inline const T Node::as() const {
 template <typename T, typename S>
 inline const T Node::as(const S& fallback) const {
   if (!m_isValid)
-    throw InvalidNode();
+    return fallback;
   return as_if<T, S>(*this)(fallback);
 }