nathelper: Fixed memory leak in nathelper fetch contact timer (GH #2062)

- Allocated buffer is not freed when contact fetch is failed
- Leak frequently occur when database disconnect and reconnect

src/modules/nathelper/nathelper.c

@@ -2003,6 +2003,8 @@ static void nh_timer(unsigned int ticks, void *timer_idx)
 					+ iteration,
 			natping_processes * natping_interval, options);
 	if(rval < 0) {
+		if(buf != NULL)
+			pkg_free(buf);
 		LM_ERR("failed to fetch contacts\n");
 		goto done;
 	}