lavf/mp3enc: don't abort if audio packets can't be buffered

Instead skip the picture streams and flush the queued audio packets.
Should address ticket #3851.

Signed-off-by: James Almer <jamrial@gmail.com>
bktr: Fix Fabrice's name

Signed-off-by: Timothy Gu <timothygu99@gmail.com>

This file with the incorrect name was added after the name was fixed in all other files.
This is thus fixing a mistake

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>