Add a missing check for the number of children before accessing them.

parentDemangling is going to be added as the first child of the return node, so
we can't construct one unless we have both children.

rdar://77530286

Author: adrian-prantl

include/swift/Remote/MetadataReader.h

@@ -2477,6 +2477,9 @@ class MetadataReader {
     // Use private declaration names for anonymous context references.
     if (parentDemangling->getKind() == Node::Kind::AnonymousContext
         && nameNode->getKind() == Node::Kind::Identifier) {
+      if (parentDemangling->getNumChildren() < 2)
+        return nullptr;
+
       auto privateDeclName =
         dem.createNode(Node::Kind::PrivateDeclName);
       privateDeclName->addChild(parentDemangling->getChild(0), dem);