scanner: use zxalloc

Use xzalloc instead of xmalloc. This allows us to get rid
of manual initializing the memory to 0s and the code is
shorter and cleaner

Suggested by Bryce Harrington

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>