Fix crash in twin_composite if msk == NULL
Fix crash in twin_composite if msk == NULL

Oops, introduced by the transform changes, I wasn't testing
msk for NULL before dereferencing it in one spot.


Signed-off-by: Benjamin Herrenschmidt <a7089bb6e7e92505d88aaff006cbdd60cc9120b6@kernel.crashing.org>
freedesktop-unofficial-mirror/twin,freedesktop-unofficial-mirror/twin