Check for illegal NULL-pointer in crypto engine

[jovanbulck]

Currently, sancus_wrap assumes some arguments (e.g., associated data) are valid non-NULL pointers. This can result in weird bugs from 0x0+offset memory writes.

While this is not a security issue, as trusted intra-SM code should never invoke sancus_wrap with incorrect arguments, and crypto memory accesses are subject to the usual memory protection checks, we could easily detect illegal arguments and fail with an error code return value.