jack: Do not Xrun the ALSA buffer

jack: Do not Xrun the ALSA buffer

when the JACK thread is requesting too many audio frames

Playback:
Without this commit the ALSA audio buffer will be played with endless
repeats as long as the user application has not provided new audio data.
Therefore this garbage will be played as long as the user application has
not called snd_pcm_stop() after an Xrun. With this fix the rest of the
JACK buffer will be filled with silence.

Capture:
Without this commit the audio data in the ALSA buffer would be
overwritten. With this commit the new data from the JACK buffer will not
be copied. Therefore the existing data in the ALSA buffer will not be
overwritten.

Signed-off-by: Timo Wischer <twischer@de.adit-jv.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>