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

audio/audio.c

@@ -2028,6 +2028,8 @@ void AUD_del_capture (CaptureVoiceOut *cap, void *cb_opaque)
                     sw = sw1;
                 }
                 QLIST_REMOVE (cap, entries);
+                g_free (cap->hw.mix_buf);
+                g_free (cap->buf);
                 g_free (cap);
             }
             return;