ff_av1_filter_obus_buf() and ff_avc_parse_nal_units_buf() both have a
pointer-to-pointer parameter which they use to pass a newly allocated
buffer to the caller. And both functions freed what this pointer points to
before overwriting it. But no caller of these functions used this feature,
but some had to initialize the pointer just because of this. So remove
it and update the documentation of ff_av1_filter_obus_buf() wrt this fact.

ff_hevc_annexb2mp4_buf in contrast did not free the pointer. This has been
documented, too.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>