patch 9.0.2134: ml_get error when scrolling

patch 9.0.2134: ml_get error when scrolling

Problem:  ml_get error when scrolling after delete
Solution: mark topline to be validated in main_loop
          if it is larger than current buffers line
          count

reset_lnums() is called after e.g. TextChanged autocommands and it may
accidentally cause curwin->w_topline to become invalid, e.g. if the
autocommand has deleted some lines.

So verify that curwin->w_topline points to a valid line and if not, mark
the window to have w_topline recalculated in main_loop() in
update_topline() after reset_lnums() returns.

fixes: #13568
fixes: #13578

Signed-off-by: Christian Brabandt <cb@256bit.org>