avformat/au: Remove redundant av_freep()

avformat/au: Remove redundant av_freep()

This av_freep(&key) in conjunction with the fact that the loop condition
checks for key != NULL was equivalent to a av_freep(&key) + a break
immediately thereafter. But given that there is an av_freep(&key)
directly after the loop, the av_freep(&key) is unnecessary and the break
can also be added explicitly.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>