avfilter/af_afir: Fix leak of AVFilterChannelLayout in case of error

avfilter/af_afir: Fix leak of AVFilterChannelLayout in case of error

If an error happens between the allocation of an AVFilterChannelLayout
and its usage (which involves attaching said object to a more permanent
object), the channel layout array leaks. This can simply be fixed by
making sure that nothing is between the allocation and the
aforementioned usage.

Fixes Coverity issue #1250334.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>