utils: fix avcodec_flush_buffers pre-reference counting compatibility

The to_free AVframe must be freed just like the other ones.
Indeed, the calling application may expect all frames to be
released.

(This regression caused use-after-free in VLC with hwaccel.)

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>