ctdb_io: fix use-after-free on invalid packets

ctdb_io: fix use-after-free on invalid packets

Wolfgang saw a talloc complaint about using freed memory in ctdb_tcp_read_cb.
His fix was to remove the talloc_free() in that function, which causes
loops when a socket is closed (as it does not get removed from the event
system), eg:
	netcat 192.168.1.2 4379 < /dev/null

The real bug is that when we have more than one pending packet in the
queue, we loop calling the callback without any safeguards should that
callback free the queue (as it tends to do on invalid packets).  This
can be reproduced by sending more than one bogus packet at once:
	# Length word at start: 4 == empty packet (assumed little endian)
	/usr/bin/printf \\4\\0\\0\\0\\4\\0\\0\\0 > /tmp/pkt
	netcat 192.168.1.2 4379 < /tmp/pkt

Using a destructor we can check if the callback frees us, and exit
immediately.  Elsewhere, we return after the callback anyway.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>