Hopefully a better fix for the paste and clipboard paste crashers. Also a bit of reformatting of the indentation inside one of the functions that did not have consistent indentation. LATER: figure out why the front-end is not disabling options like paste at start-up.

We were erroneously using canvas_getrootfor instead of glist_getcanvas to
find the canvas on which the nested gop was being displayed.

Also added a consistency check to turn such a crasher into a runtime bug
in the future.