Add missing acquire memory barrier in isc_nmhandle_unref

The ThreadSanitizer uses system synchronization primitives to check for
data race.  The netmgr handle->references was missing acquire memory
barrier before resetting and reusing the memory occupied by isc_nmhandle_t.

lib/isc/netmgr/netmgr.c

@@ -1156,6 +1156,8 @@ isc_nmhandle_unref(isc_nmhandle_t *handle) {
 	if (isc_refcount_decrement(&handle->references) > 1) {
 		return;
 	}
+	/* We need an acquire memory barrier here */
+	(void)isc_refcount_current(&handle->references);
 
 	sock = handle->sock;
 	handle->sock = NULL;