api/window: validate cursor in nvim_win_set_buf

validate_cursor() is called regularly, but only for the current window.
When changing the buffer for a non-current window, we need to invoke it
in the context of that window.

src/nvim/api/window.c

@@ -67,6 +67,10 @@ void nvim_win_set_buf(Window window, Buffer buffer, Error *err)
                   buffer);
   }
 
+  // If window is not current, state logic will not validate its cursor.
+  // So do it now.
+  validate_cursor();
+
   restore_win(save_curwin, save_curtab, false);
 }