peddi crashes or misdisplays near EOL or EOF

[pzembrod]

Repro:
Remove the shift-return (\x8d) at the beginning of line 21 of editfile.keybuf, and the editfile test will crash peddi.
I'm not clear what the problem exactly is, but it seems to be connected to long lines or horizontal scrolling to the end of such lines, or vertical srolling when horizontally scrolled to the end of a long line. The crash seems to triggered by the kill-line command (ctrl-P, \x10) in line 25 of editfile.keybuf.

Possibly related: When one scrolls down to the end of the test file editfile.before (or widelines.before), then the screen displays a line of tilde characters at some point, which disappears again when one scrolls to the top again. Not sure if this is just a display bug or if it potentially affects the file content as well. It doesn't seem to affect the file content in the above scenario, but it might if one makes edits e.g. during the bad display state.

Better testing would be desirable for this, so far I don't have a good way of verifying the screen display in an automated test.