Fix corruption of an island mask in assign_leftovers

[AlexKurek]

Mask updating
Replaced the incorrect reassignment (mask = (mlabels == ii)) with proper in-place pixel masking (mask[tuple(cc)] = True). (There was a commented-out attempt # mask[cc] = True indicating the original intent).

Loop termination
The return was executed too early, meaning that not all pixels could be processed.

[gmloose]

You removed the return statement, which changes the logic of the function. It will now continue into the else branch of if len(belongs) == 1, which is probably not what you want. OTOH, I find the original method confusing in the sense that it either returns None or a list. I'm not sure why it doesn't simply return the unaltered labels in this case, which I think it should. @darafferty, what do you think?

[AlexKurek]

I agree, I have restored return.

[gmloose]

Yeah, well the question remains if it should return labels, which is provided as input and will most likely not be None, or None as it was. Like I said, I'm confused about the original code, so this might as well be a bug fix.