samples: net: Fix out-of-bound access

Payload variable can hold max 64 bytes but value of size can be more
than that. Memsetting using value of size can overrun payload array.
Fixes coverity issue.

Coverity-CID: 187823
Fixes: #9638

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>