If adding an SEI message to an access unit fails, said SEI message was
not touched, so that the caller had to free any data associated with it
that might need to be freed. But given that ff_cbs_h264_add_sei_message
can simply call cbs_h264_free_sei_payload, one can easily free
the content of the SEI payload.

This fixes a memleak when inserting a user data unregistered string for
h264_metadata fails.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>