Don't use NULL s->server_hostname as hash lookup key.

Reported by paritosh (at limewire.co.in) on dev@httpd (thread
Re: apache 2.2 crashes at the start time in mod_dbd.c then preparing AuthDBDUserPWQuery)
Fixed by paritosh and Yours Truly.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@432560 13f79535-47bb-0310-9956-ffa450edef68