shell: don't assign output for surface of type none

If map is called with a surface of type none it will call
weston_surface_assign_output, even though the surface will
not be mapped.

src/shell.c

@@ -1427,6 +1427,7 @@ map(struct weston_shell *base, struct weston_surface *surface,
 		do_configure = 0;
 		break;
 	case SHELL_SURFACE_NONE:
+		do_configure = 0;
 		break;
 	default:
 		/* everything else just below the panel */