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.

This is a backport of I488d0207494e3a3fad52e64e67b2e740b236b864

BUG: 1138897
Change-Id: I001ab73b37f74ba98463bd9c32c01c670e9c7ad8
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8629
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>