[mcedit] Text block deletion moves cursor far after the end of line

[sergem]

Steps to reproduce:

1. Shift+F4, an empty editor starts
2. On the first line type characters to almost hit the right side of the screen (i.e. if console is 80x25 type 75 characters on the first line), then press Enter.
3. On the second line type a few characters and then move cursor to the end of the first line (Up, End).
4. Now press: Left, F3, Down, F8

Expected result:
Selection part deleted, and cursor is right after the last character of the first line.

Actual result:
Cursor jumps far after the end of the first line and shifts screen right.