Fix check for failed evdns request creation

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 81eee0ecfff3dac1e9438719d2f7dc0ba7e84a71; found by
lodger; uploaded to trac by rransom.  Bug 2363.  Fix on 0.2.2.6-alpha.