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>

src/window.c

@@ -2475,6 +2475,7 @@ win_init_empty(win_T *wp)
     wp->w_topfill = 0;
 #endif
     wp->w_botline = 2;
+    wp->w_valid = 0;
 #if defined(FEAT_SYN_HL) || defined(FEAT_SPELL)
     wp->w_s = &wp->w_buffer->b_s;
 #endif