Fix: Crash when muting the Group Output node

This fixes a crash when muting the "Group Output" node.
It should not be possible to mute it so this patch
sets the `no_muting`-variable on it.

Differential Revision: https://developer.blender.org/D13364

source/blender/nodes/intern/node_common.cc

@@ -613,6 +613,8 @@ void register_node_type_group_output(void)
   node_type_init(ntype, node_group_output_init);
   node_type_update(ntype, node_group_output_update);
 
+  ntype->no_muting = true;
+
   nodeRegisterType(ntype);
 }