Although the minimum size of ninepatches is set to the sum of the margins in normal use (through gdscript etc) it turns out that it is possible to programmatically create ninepatches that are small than this minimum - in particular zero size is used in sliders to not draw items.
This PR deals with zero sized ninepatches by not drawing anything, and has some basic protection for ninepatches smaller than the margins. Whether these occur in the wild is not clear but is put in for completeness.
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