Allocating coded_frame is what most encoders do anyway, so it makes
sense to always allocate and free it in a single place. Moreover a lot
of encoders freed the frame with av_freep() instead of the correct API
av_frame_free().

This bring uniformity to encoder behaviour and prevents applications
from erroneusly accessing this field when not allocated. Additionally
this helps isolating encoders that export information with coded_frame,
and heavily simplifies its deprecation.
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>