global dns cache: fix memory leak

The take down of the global dns cache didn't take CURLOPT_RESOLVE names
into account.

lib/hostip.c

@@ -140,6 +140,10 @@ struct curl_hash *Curl_global_host_cache_init(void)
 void Curl_global_host_cache_dtor(void)
 {
   if(host_cache_initialized) {
+    /* first make sure that any custom "CURLOPT_RESOLVE" names are
+       cleared off */
+    Curl_hostcache_clean(NULL, &hostname_cache);
+    /* then free the remaining hash completely */
     Curl_hash_clean(&hostname_cache);
     host_cache_initialized = 0;
   }