rtph265pay: don't add trailing zeros to VPS/PPS/SPS

rtph265pay: don't add trailing zeros to VPS/PPS/SPS

This would happen if input is byte-stream with four-byte
sync markers instead of three-byte ones. The code that
scans for sync markers will place the start of the NALU
on the third-last byte of the NALU sync marker, which
means that any additional zeros may be counted as belonging
to the previous NALU instead of being part of the next sync
marker. Fix that so we don't send VPS/SPS/PPS with trailing
zeros in this case.

See https://bugzilla.gnome.org/show_bug.cgi?id=732758