Fix check for failed evdns request creation
When using libevent 2, we use evdns_base_resolve_*(). When not, we fake evdns_base_resolve_*() using evdns_resolve_*(). Our old check was looking for negative values (like libevent 2 returns), but our eventdns.c code returns 1. This code makes the check just test for nonzero. Note that this broken check was not for _resolve_ failures or even for failures to _launch_ a resolve: it was for failures to _create_ or _encode_ a resolve request. Bug introduced in 81eee0ec; found by lodger; uploaded to trac by rransom. Bug 2363. Fix on 0.2.2.6-alpha.