shell: Don't ping internal shell surfaces

This is a hack for now to get xserver surfaces working again.
diff --git a/src/shell.c b/src/shell.c
index ed5ca80..fd66462 100644
--- a/src/shell.c
+++ b/src/shell.c
@@ -418,6 +418,8 @@
 
 	if (!shsurf)
 		return;
+	if (!shsurf->resource.client)
+		return;
 
 	if (!shsurf->ping_timer) {
 		shsurf->ping_timer = malloc(sizeof shsurf->ping_timer);