xwayland: Fix memory allocation test

Due to a typo in tablet_pad_group(), we would allocate a variable
("group") and test another one ("pad") for allocation success.

Spotted by covscan.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Fixes: commit 8475e63 - "xwayland: add tablet pad support"
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>

hw/xwayland/xwayland-input.c

@@ -2268,7 +2268,7 @@ tablet_pad_group(void *data,
     struct xwl_tablet_pad_group *group;
 
     group = calloc(1, sizeof *group);
-    if (pad == NULL) {
+    if (group == NULL) {
         ErrorF("%s ENOMEM\n", __func__);
         return;
     }