pty_chr_timer first calls pty_chr_update_read_handler(), then clears
timer_tag (because it is a one-shot timer). This is the wrong order
though. pty_chr_update_read_handler might re-arm time timer, and the
new timer_tag gets overwitten in that case.
This leads to crashes when unplugging a pty chardev: pty_chr_close
thinks no timer is running -> timer isn't canceled -> pty_chr_timer gets
called with stale CharDevState -> BOOM.
This patch fixes the ordering.
Kill the pointless goto while being at it.