ff_hevc_annexb2mp4_buf() could indicate an error, yet leave cleaning
after itself to the caller, so that a caller could not simply return the
error, but had to free the buffer first.

(Given that all current callers have set filter_ps = 0, this error can
currently not be triggered.)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>