userspace: Additional checks in K_SYSCALL_MEMORY

This macros needed additional checks before invoking
arch_buffer_validate.

- size can not be less then 0. Some functions invoke this macro
  using signed type which will be promote to unsigned when invoking
  arch_buffer_validate. We need to do an early check.
- We need to check for possible overflow, since a malicious user
  application could use a negative number that would be promoted
  to a big value that would cause a integer overflow when adding it
  to the buffer address, leading to invalid checks.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>