Handle the situation where the ng iterators get an empty stretch

Assert in developer mode so we know where to add the check for emptiness
before creating the iterator; make the iterator check at least a pixel
in release mode.

We cannot re-instate the old isDone() api because that would impair
performance too much:

13:26:45 < boud> CyrilleB: do you remember why the NG iterators don't have an isDone() check so the loop always checks at least once? (with do {} while
                 (it-NextPixel())
13:45:34 < CyrilleB> boud: performance
13:46:13 < CyrilleB> boud: especially since in 99.99% of the case, one does have something to iterate over
13:50:34 < boud> aah
13:50:49 < boud> because it's the reason of the crash in KisSelection::invert
13:52:29 < boud> maybe if the rect is empty we should make it have a width/height of one so there's always one pixel read in all cases
13:53:04 < CyrilleB> or rather have an assert if you try to create an iterator with a null width/height
13:53:20 < CyrilleB> (or both, if you want to ensure good behaviour for users)
                    it goes wrong
13:53:35 < CyrilleB> s/good behaviour/non crashing behaviour/