pcm_file: in case of failed write clear file buffer variables

pcm_file: in case of failed write clear file buffer variables

previously, in case of failed write to output file, error is returned
from snd_pcm_writei/read APIs, user could run pcm_drain as fallback and
encounter an assert, since drain would try to write remaining file
buffer to a file

if failed to write to output file in first place, it makes sense to clear
current internal pmc_file file buffer variables

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