Fix an error for debug locking in dns/getaddrinfo_race_gotresolve

Fix an error for debug locking in dns/getaddrinfo_race_gotresolve

When there is no /etc/services file evdns_getaddrinfo() will fail (with
service="ssh") and hence it will go to then "end" label with locked
rp.lock which in case of debug locking checks will bail with:
  [err] ../evthread.c:220: Assertion lock->count == 0 failed in debug_lock_free

So add rp.locked flag, and unlock the lock before freeing it if it is in
locked state.

And here is how you can reproduce the issue:
  $ docker run -e LD_LIBRARY_PATH=$PWD/lib -e PATH=/usr/bin:/bin:$PWD/bin -v $PWD:$PWD --rm -it debian:testing regress dns/getaddrinfo_race_gotresolve
(since debian:testing does not have /etc/services)