Fix UAF in ccmsg.c when reading stopped before sending

Fix UAF in ccmsg.c when reading stopped before sending

When shutting down the whole server, the reading could stop and detach
from controlconnection before sending is done.  If send callback then
detaches from the last controlconnection handle, the ccmsg would be
invalidated after the send callback and thus we must not access ccmsg
after calling the send_cb().