xwayland/shm: Use `calloc()`

Currently, Xwayland pixmap SHM code uses `malloc()` to allocate the
xwl_pixmap.

Use `calloc()` instead, as the EGLstream backend does, as it is safer
(initializing the allocated data to 0).

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>

hw/xwayland/xwayland-shm.c

@@ -210,7 +210,7 @@ xwl_shm_create_pixmap(ScreenPtr screen,
     if (!pixmap)
         return NULL;
 
-    xwl_pixmap = malloc(sizeof *xwl_pixmap);
+    xwl_pixmap = calloc(1, sizeof(*xwl_pixmap));
     if (xwl_pixmap == NULL)
         goto err_destroy_pixmap;