This code uses the AVBPrint API for exactly one av_bprintf()
in a scenario in which a good upper bound for the needed
size of the buffer is available (with said upper bound being
much smaller than sizeof(AVBPrint)). So one can simply use
snprintf() instead. This also avoids the (always-false due to
the current size of the internal AVBPrint buffer) check for
whether the AVBPrint is complete.

Furthermore, the old code used AV_BPRINT_SIZE_AUTOMATIC
which implies that the AVBPrint buffer will never be
(re)allocated and yet it used av_bprint_finalize().
This has of course also been removed.
Reviewed-by: Jan Ekström <jeebjp@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>