[Image-SIG] Converting images to palette mode

[Fredrik Lundh]

It's the same fix as this one, right?

http://bitbucket.org/effbot/pil-2009-raclette/changeset/577ae0c2a85e/

I haven't been able to force the leak during testing (the loop can
terminate in several ways, and only one of them leaks memory; none of
my test images seemed to trigger this behaviour with the test program
I used), but the fix is "obviously correct" so it's in.

</F>