V4L/DVB: Fix the risk of an oops at dvb_dmx_release

dvb_dmx_init tries to allocate virtual memory for 2 pointers: filter and feed.

If the second vmalloc fails, filter is freed, but the pointer keeps pointing
to the old place. Later, when dvb_dmx_release() is called, it will try to
free an already freed memory, causing an OOPS.

Reviewed-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>