V4L/DVB (9920): em28xx: fix NULL pointer dereference in call to VIDIOC_INT_RESET command

Fix a NULL pointer dereference that would occur if the video decoder tied to
the em28xx supports the VIDIOC_INT_RESET call (for example: the cx25840 driver)

Signed-off-by: Devin Heitmueller <dheitmueller@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>