hostip: don't remove DNS entries that are in use

hostcache_timestamp_remove() should remove old *unused* entries from the
host cache, but it never checked whether the entry was actually in
use. This complements commit 030a2b8c.

Bug: http://curl.haxx.se/bug/view.cgi?id=1327

lib/hostip.c

@@ -237,7 +237,7 @@ hostcache_timestamp_remove(void *datap, void *hc)
     (struct hostcache_prune_data *) datap;
   struct Curl_dns_entry *c = (struct Curl_dns_entry *) hc;
 
-  return (data->now - c->timestamp >= data->cache_timeout);
+  return !c->inuse && (data->now - c->timestamp >= data->cache_timeout);
 }
 
 /*