Ensure the correct ordering zone_shutdown() vs zone_gotwritehandle()

Ensure the correct ordering zone_shutdown() vs zone_gotwritehandle()

When the signed version of an inline-signed zone is dumped to disk, the
serial number of the unsigned version of the zone is written in the
raw-format header so that the contents of the signed zone can be
resynchronized after named restart if the unsigned zone file is
modified while named is not running (see RT #26676).

In order for the serial number of the unsigned zone to be determined
during the dump, zone->raw must be set to a non-NULL value.  This
should always be the case as long as the signed version of the zone is
used for anything by named.

However, under certain circumstances the zone->raw could be set to NULL
while the zone is being dumped.

Defer detaching from zone->raw in zone_shutdown() if the zone is in the
process of being dumped to disk.

(cherry picked from commit cc1d4e1aa63fdc7e82a00fd7b06d50fb4dbc8e96)