media/dvb: Add checks if transport stream is existing

Chances of kernel crash when CAS clients call DMX_STOP without
successful prior DMX_START is eliminated by checking for a valid TS.

Change-Id: I760cdde238b076d1853afb1a3943de84bc88d96e
Signed-off-by: Murthy Nidadavolu <mnssr@codeaurora.org>