ctdb->methods becomes NULL when we shutdown the transport.

ctdb->methods becomes NULL when we shutdown the transport.

If we shutdown the transport   and CTDB later decides to send a command out
for queueing, the call to ctdb->methods->allocate_pkt() will SEGV.

This could trigger for example when we are in the process of shuttind down CTDBD and have already shutdown the transport but we are still waiting for the
"shutdown" eventscripts to finish.
If the event scripts now take much much longer to execute for some reason, this
race condition becomes much more probable.

Decorate all dereferencing of ctdb->methods->    with a check that ctdb->menthods is non-NULL