media: dvb: check filter is valid for fullness callback

In PULL mode, all relevant filters' buffer space is checked
to ensure sufficient space or otherwise blocking untill it
becomes available. As callback might sleep it is called without
holding any locks. So at this time, just before the callback executes,
the filter might have been closed by a different thread.
To prevent invalid access, verify in the callback that filter is still
valid.

Change-Id: Ie07ac5d17942ef39d12e4eed3078e382c956fe2e
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>