glibpacrunner: fix crash on error

glibpacrunner was asserting that g_proxy_resolver_lookup() didn't
return an error (since libproxy 0.3 always returned a non-error
response no matter what happened). But libproxy 0.4 does error out and
return NULL in some rare cases, so we need to allow for that.

https://bugzilla.redhat.com/show_bug.cgi?id=866927