xwayland/glamor-gbm: Use `calloc()`

Currently, glamor GBM backend 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-glamor-gbm.c

@@ -149,7 +149,7 @@ xwl_glamor_gbm_create_pixmap_for_bo(ScreenPtr screen, struct gbm_bo *bo,
     struct xwl_pixmap *xwl_pixmap;
     struct xwl_screen *xwl_screen = xwl_screen_get(screen);
 
-    xwl_pixmap = malloc(sizeof *xwl_pixmap);
+    xwl_pixmap = calloc(1, sizeof(*xwl_pixmap));
     if (xwl_pixmap == NULL)
         return NULL;