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After a call to the Formatter or the Alternate spell checker, the data
on the undo stack could refer to line positions that don't exist any
more -- the chance is small, but it is there.  So... throw the whole
undo stack away, to prevent undoing things wrongly or even crashing.

(Changes made with the internal spell checker can be undone and redone
without a problem -- nano keeps full track of those.  But the changes
made with a formatter or an external spell checker happen in another
process, so nano has no record of them.)

This fixes https://savannah.gnu.org/bugs/?49920.