h264parser/h264parse: Handle trailing 0s in NALU

Submitted by Edward Hervey @bilboed

Link to original bug (#747850)

Description

the H264 spec allows to have leading/traling zeros in byte stream

The problem is that we don't know about it, and therefore we iterating over
NALU we are never guaranteed to end at the next NALU start code, but instead
on potentially trailing zero bytes

This would cause h264parse to tell baseparse to skip those few bytes, resulting
in the next buffer being a DISCONT one ... whereas it's not.