channels: Avoid memory leak

Thanks oss-fuzz:

https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=40116

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/channels.c

@@ -245,6 +245,7 @@ SSH_PACKET_CALLBACK(ssh_packet_channel_open_fail){
               "SSH2_MSG_CHANNEL_OPEN_FAILURE received in incorrect channel "
               "state %d",
               channel->state);
+      SAFE_FREE(error);
       goto error;
   }