Use primary router when sending WHOIS packet if client->router is NULL.

client may be non-NULL, but client->router may be NULL when we enter
silc_server_query_client in the case of a detached client on a remote
server that has not been fully resolved yet.  In this case, we should
try and send the packet using the primary router.  This fixes a crash
wherein silc_server_query_client would sometimes crash with a NULL
dereference if called on a remote detached client.