An AVBufferRef (and the corresponding AVBuffer and the underlying actual
buffer) would leak in ff_cbs_sei_add_message() on error in case an error
happened after its creation and before it has been attached to more
permanent storage. Fix this by only creating the AVBufferRef immediately
before attaching it to its intended target position.

(Given that no SEI message currently created is refcounted, the above
can't happen at the moment. But Coverity already nevertheless noticed:
This commit fixes Coverity issue #1473521.)
Reviewed-by: Mark Thompson <sw@jkqxz.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>