abort snd_pcm_dmix_drain when state is SND_PCM_STATE_XRU

abort snd_pcm_dmix_drain when state is SND_PCM_STATE_XRU
From: Sascha Sommer <saschasommer@freenet.de>

this is a fix for bug 0001559.
Unlike my first guesses the real problem is not in the kernel driver but in
alsa-lib. Whenever the current dmix status is xrun and snd_pcm_dmix_drain
gets called the process will hang forever in the poll function. The reason is
that poll gets called even though the timer already stopped.
As described in the bugtracking system this bug was not noticable with alsa
versions that used the old IOCTLS because the SNDRV_TIMER_IOCTL_STOP ioctl
never reached the timer kernel module.

Attached patch fixes this problem for alsa-lib 1.0.10 by simply calling
snd_pcm_dmix_drop when snd_pcm_dmix_drain gets called in the state
SND_PCM_STATE_XRUN.