Fix segfault in a child process.

Fix segfault in a child process.

It is reported that pgpool child segfaulted [1].

[snip]

In the down thread it is reported that despite VALID_BACKEND(i)
returns true, backend->slots[i] is NULL, which should have been filled
by new_connection().

It seems there's a race condition. In new_connection(), there's a code
fragment:
		/*
		 * Make sure that the global backend status in the shared memory
		 * agrees the local status checked by VALID_BACKEND. It is possible
		 * that the local status is up, while the global status has been
		 * changed to down by failover.
		 */
A-->		if (BACKEND_INFO(i).backend_status != CON_UP &&
			BACKEND_INFO(i).backend_status != CON_CONNECT_WAIT)
		{
			ereport(DEBUG1,
					(errmsg("creating new connection to backend"),
					 errdetail("skipping backend slot %d because global backend_status = %d",
							   i, BACKEND_INFO(i).backend_status)));

			/* sync local status with global status */
B-->			*(my_backend_status[i]) = BACKEND_INFO(i).backend_status;
			continue;
		}

It is possible that at A backend_status in the shared memory is down
but by the time it reaches B the status has been changed to up. And
new_connection() skipped to create a backend connection. This seems to
explain why the connection slot is NULL while VALID_BACKEND returns
true. To prevent the race condtion, backend_status in shared memory is
copied to a local variable and evaluate it.  Also the VALID_BACKEND
just before:

	pool_set_db_node_id(CONNECTION(backend, i), i);

is changed to:
			if (VALID_BACKEND(i) && CONNECTION_SLOT(backend, i))

so that it prevents crash just in case.

[1] [pgpool-general: 9104] Another segmentation fault