shell: don't map surfaces with type NONE

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
diff --git a/src/shell.c b/src/shell.c
index da6e6ac..78db0cb 100644
--- a/src/shell.c
+++ b/src/shell.c
@@ -1421,6 +1421,8 @@
 		}
 		do_configure = 0;
 		break;
+	case SHELL_SURFACE_NONE:
+		break;
 	default:
 		/* everything else just below the panel */
 		if (!wl_list_empty(&shell->panels)) {