mp_image: fix vf_vdpaupp references

Some code called by vf_vdpaupp.c calls mp_image_new_custom_ref(), but
out of convenience doesn't reset the buffers. Make this behavior ok.
(The assert() was there to catch usage errors, but the same error could
already happen before the refcount changes were made, so the check is
not overly helpful.)

Fixes #2115.

video/mp_image.c

@@ -241,8 +241,6 @@ struct mp_image *mp_image_new_dummy_ref(struct mp_image *img)
 struct mp_image *mp_image_new_custom_ref(struct mp_image *img, void *free_arg,
                                          void (*free)(void *arg))
 {
-    assert(!img->bufs[0]);
-
     struct mp_image *new = mp_image_new_dummy_ref(img);
 
     struct free_args *args = talloc_ptrtype(NULL, args);