ao_alsa: Sanity check get_space() return values better

ao_alsa: Sanity check get_space() return values better

Sometimes after seek audio reset I see snd_pcm_status_get_avail()
return huge values. get_space() already had a check againt the value
being larger than the whole buffer; however since the unsigned value
from the ALSA function had been cast to signed by that point it was
interpreted as negative and the check didn't trigger. Use unsigned
instead to make the check reliable and ensure the return value is sane.