Fix a hard-to-trigger memory leak in launch_resolve

To hit this leak, you need to be a relay that gets a RESOLVE request
or an exit node getting a BEGIN or RESOLVE request.  You must either
have unconfigured (and unconfigurable) nameservers, or you must have
somehow set DisableNetwork after a network request arrived but
before you managed to process it.

So, I doubt this is reached often.  Still, a leak's a leak.  Fix for
bug 5916; bugfix on 0.2.3.9-alpha and 0.1.2.1-alpha.