media: dvb: Add check for 'orphan' headers on S-PES header pipe

When overflow occurs on payload pipe headers are still written
to the header pipe but cannot point to the payload that was
discarded. Add check for such headers and discard them.

Change-Id: I6a113a270ec33b6ce595030591e647f1f5318897
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>