In previous state, a new frame was allocated on each timestamp step,
i.e. each frame/field transition. However, for interlace, a new frame
should be allocated on 1st field, completed with the 2nd and finally
freed.

This commit fixes the frame allocation and the detection of missing RTP
markers.
Signed-off-by: Patrick Keroulas <patrick.keroulas@radio-canada.ca>
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>