Fix null dereference when loading schematic from definition without a NodeDefManager

src/script/lua_api/l_mapgen.cpp

@@ -215,6 +215,7 @@ Schematic *load_schematic_from_def(lua_State *L, int index,
 		}
 	}
 
+	if (ndef)
 		ndef->pendNodeResolve(schem);
 
 	return schem;