Discarding (in commit 6f9bb53b) the cap on the number of chunks to
move backwards had as an unforeseen side effect that the screen can
fail to scroll when the cursor is somehow pushed offscreen.

Fix this by setting the target row (for smooth scrolling) always to
the bottom row of the edit window when nano notices that the cursor
has gone offscreen.

This fixes https://savannah.gnu.org/bugs/?51676

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Reported-by: David Lawrence Ramsey <pooka109@gmail.com>