lock the DNS cache properly before adding an entry when using asynch DNS

lib/hostip.c

@@ -527,8 +527,16 @@ static void host_callback(void *arg, /* "struct connectdata *" */
       /* pack_hostent() copies to and shrinks the target buffer */
       struct hostent *he = pack_hostent(&bufp, hostent);
 
-      dns = cache_resolv_response(conn->data, he,
+      struct SessionHandle *data = conn->data;
+
+      if(data->share)
+        Curl_share_lock(data, CURL_LOCK_DATA_DNS, CURL_LOCK_ACCESS_SINGLE);
+
+      dns = cache_resolv_response(data, he,
                                   conn->async.hostname, conn->async.port);
+
+      if(data->share)
+        Curl_share_unlock(data, CURL_LOCK_DATA_DNS);
     }
   }