Do not call rpc_transport_unref() on NULL trans

rpc_clnt_disable() sets rpc->conn->trans to NULL, hence we should not
call rpc_transport_unref() afterwards. I moved it before the
rpc_clnt_disable() call, but I am not sure it should be called at all,
perhaps it should just go away.

BUG: 764655
Change-Id: I488d0207494e3a3fad52e64e67b2e740b236b864
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8393
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Tested-by: Raghavendra G <rgowdapp@redhat.com>