Prevent invalid image array indexing

Summary:
Fixed cases mainly come from the fact that ScreenWindow can be
larger that TerminalDisplay's image, and the way how selection area is
bounded (character's left edge, not first/last character in selection).

Test Plan:

• Compile with ASAN
• Turn on blinking cursor
• Slowly change window size to less than 1 line or less than 1 column
• If everything is still ok, run top or anything that generates longer output

Expected result: no overflows

• Random selections (normal/block/line/word):
  • on screen in left/right direction
  • on screen + in history
    • selecting history up
    • selecting history down
  • first/last character → last/first character in the line
  • first/last character → last/first character on the screen
  • all above with wide characters

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: konsole-devel, hindenburg, Konsole

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D12551