media: video-mux: fix null pointer dereferences

commit aeb0d0f5 upstream.

devm_kcalloc may fail and return a null pointer. The fix returns
-ENOMEM upon failures to avoid null pointer dereferences.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>