patch 9.1.0374: wrong botline in BufEnter

patch 9.1.0374: wrong botline in BufEnter

Problem:  When :edit an existing buffer, line('w$') may return a
          wrong result.
Solution: Reset w_valid in curwin_init() (Jaehwang Jung)

`do_ecmd()` reinitializes the current window (`curwin_init()`) whose
`w_valid` field may have `VALID_BOTLINE` set. Resetting `w_botline`
without marking it as invalid makes subsequent `validate_botline()`
calls a no-op, thus resulting in wrong `line('w$')` value.

closes: #14642

Signed-off-by: Jaehwang Jung <tomtomjhj@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>