patch 9.1.1823: diff: w_topline may be invalidated

patch 9.1.1823: diff: w_topline may be invalidated

Problem:  diff: w_topline may be invalidated
Solution: Update lnum in diff_set_topline()
          (Yee Cheng Chin).

This can happen in ex_diffupdate() for certain edge cases which cause
the logic to now be wrong. This was also the root cause for #18437 where
Vim would crash due to a null pointer dereferencing (said pointer would
not be null under normal circumstances).

related: #18437
closes: #18484

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>