ALSA: compress: Memset timestamp structure to zero.

snd_compr_tstamp is initialized using aggregate initialization
that does not zero out the padded bytes. Initialize timestamp
structure to zero using memset to avoid this.

CRs-Fixed: 568717
Change-Id: I7a7d188705161f06201f1a1f2945bb6acd633d5d
Signed-off-by: Krishnankutty Kolathappilly <kkolat@codeaurora.org>