patch 9.1.0707: [security]: invalid cursor position may cause a crash

patch 9.1.0707: [security]: invalid cursor position may cause a crash

Problem:  [security]: invalid cursor position may cause a crash
          (after v9.1.0038)
Solution: Set cursor to the last character in a line, if it would
          otherwise point to beyond the line; no tests added, as it
          is unclear how to reproduce this.

Github Advisory:
https://github.com/vim/vim/security/advisories/GHSA-4ghr-c62x-cqfh

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>