audio: release capture buffers

AUD_add_capture() allocates two buffers which are never released.
Add the missing calls to AUD_del_capture().

Impact: Allows vnc clients to exhaust host memory by repeatedly
starting and stopping audio capture.

Fixes: CVE-2017-8309
Cc: P J P <ppandit@redhat.com>
Cc: Huawei PSIRT <PSIRT@huawei.com>
Reported-by: "Jiangxin (hunter, SCC)" <jiangxin1@huawei.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Prasad J Pandit <pjp@fedoraproject.org>
Message-id: 20170428075612.9997-1-kraxel@redhat.com
