V4L/DVB (3488): Snd_cx88_create: don't dereference NULL core

If the call to cx88_core_get returns a NULL value, it is dereferenced
by cx88_reset, and perhaps by cx88_core_put.  Spotted by the Coverity
checker.

Signed-off-by: Duncan Sands <baldrick@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>