Restart tick counting every command

Restart tick counting every command

* src/keyboard.c (command_loop_1): Reinitialize the tick count
before executing each command in the loop.
* src/xdisp.c (update_redisplay_ticks): Be more defensive to W
being NULL and to its buffer being nil.  Set
'windows_or_buffers_changed' to avoid trusting stale window data
like w->window_end_valid.