set_try_to_free_routine(NULL) means "do nothing special"

This way, the next caller that wants to disable our memory reclamation
machinery does not have to define its own do_nothing() stub.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

wrapper.c

@@ -12,6 +12,8 @@ static void (*try_to_free_routine)(size_t size) = do_nothing;
 try_to_free_t set_try_to_free_routine(try_to_free_t routine)
 {
 	try_to_free_t old = try_to_free_routine;
+	if (!routine)
+		routine = do_nothing;
 	try_to_free_routine = routine;
 	return old;
 }