Block dynamic NPCs in instances (#7429)

Fixes #7420

Thanks to @attackjom

src/map/npc.cpp

@@ -5805,7 +5805,10 @@ struct npc_data* npc_duplicate_npc_for_player( struct npc_data& nd, struct map_s
 		return nullptr;
 	}
 
-	// TODO: check maps that might forbid usage? maybe create mapflag?
+	if( map_getmapflag( sd.bl.m, MF_NODYNAMICNPC ) ){
+		// It has been confirmed that there is no reply to the client
+		return nullptr;
+	}
 
 	int16 new_x, new_y;