media: dvb: verify filter state when reusing decoder buffer

In case that user calls DMX_REUSE_DECODER_BUFFER ioctl after
the filter has stopped, feed->ts is NULL and so kernel will
dereference a NULL pointer.
Add checking of filter state to verify filter is active when
decoder buffer is reused and verify feed and feed->ts are not
NULL.

Change-Id: I6b8bb64cd7689e2f15813d3f777803df3bb0c849
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>