If our `ring_buffer` is smaller than the ALSA-PCM buffer (if the
latter has more than the 4 periods we allocate), it can happen that
the start threshold is crossed and ALSA switches to
`SND_PCM_STATE_RUNNING`, but the `ring_buffer` is empty.  In this
case, MPDD will generate silence, even though the ALSA-PCM buffer has
enough data.  This causes stuttering (#420).

This commit amends an older workaround for a similar problem (commit
e08598e7) by adding a snd_pcm_avail()
check, and only generate silence if there is less than one period of
data in the ALSA-PCM buffer.

Fixes #420