xwayland: Do not free a NULL GBM bo

Both `gbm_bo_create()` and `gbm_bo_create_with_modifiers()` can fail and
return `NULL`.

If that occurs, `xwl_glamor_gbm_create_pixmap()` will not create a
pixmap for the (NULL) GBM bo, but would still try to free the bo which
leads to a crash in mesa:

  [...]
  #7  <signal handler called>
  #8  in gbm_bo_destroy (bo=0x0) at ../src/gbm/main/gbm.c:439
  #9  in xwl_glamor_gbm_create_pixmap () at xwayland-glamor-gbm.c:245
  #10 in ProcCreatePixmap () at dispatch.c:1440
  #11 in Dispatch () at dispatch.c:478
  #12 in dix_main () at main.c:276

To avoid the crash, only free the GBM bo if not `NULL`.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Bugzilla: https://bugzilla.redhat.com/1729925
(cherry picked from commit d9ec5250)