ao_alsa: check ALSA PCM state before pause and resume

It is possible to have ao->reset() called between ao->pause() and
ao->resume() when seeking during the pause. If the underlying PCM
supports pausing, resuming an already reset PCM will produce an error.
Avoid that by explicitly checking PCM state before calling
snd_pcm_pause().

Signed-off-by: wm4 <wm4@nowhere>
ao_alsa: remove unneeded initializations

priv is 0-initialized, can_pause is always overwritten later.