Clear dns64_aaaaok immediately after use

Clear dns64_aaaaok immediately after use

The DNS64 state information stored in client->query.dns64_aaaaok
could cause an assertion failure in query_respond() if the server
was configured in such a way as to trigger a new recursion before
the query had been reset - for example, by using the filter-aaaa
plugin, which may need to recurse to find out whether an A record
exists.

This has been addressed by clearing DNS64 state information
immediately after the call to query_filter64().

(cherry picked from commit 7213b038f0beb2f4750b858113af1f9e18ae0520)