qdsp5: memset stack buffer allocation.

This is needed to avoid garbage data in uninitialized variables.

Change-Id: I3a043a4ce4e14d8e0c24b434181dff484eddac72
CRs-Fixed: 551548
CRs-Fixed: 551542
CRs-Fixed: 551555
CRs-Fixed: 551563
CRs-Fixed: 551566
Signed-off-by: Asish Bhattacharya <asishb@codeaurora.org>