Minor cleanups in X Windows drag-and-drop code

Minor cleanups in X Windows drag-and-drop code

* src/keyboard.c (kbd_buffer_get_event): Don't dereference f if
nil when generating mouse motion event.
* src/xterm.c (x_dnd_send_enter, x_dnd_send_position)
(x_dnd_send_leave, x_dnd_send_drop, x_send_scroll_bar_event):
Use NoEventMask instead of 0.