media: dvb: Protect against invalid source

If user misses setting DVR source to demux,
the source or the input pipe may be invalid
which would result in an invalid pointer access.

Change-Id: Ib80556b57ef63a9c8682bde7093cbf2d21009f4e
Signed-off-by: Hamad Kadmany <hkadmany@codeaurora.org>