Fix a data access race in resolver

Fix a data access race in resolver

We were passing client address to dns_resolver_createfetch as a pointer
and it was saved as a pointer. The client (with its address) could be
gone before the fetch is finished, and in a very odd scenario
log_formerr would call isc_sockaddr_format() which first checks if the
address family is valid (and at this point it still is), then the
sockaddr is cleared, and then isc_netaddr_fromsockaddr is called which
fails an assertion as the address family is now invalid.