We could enable additional runtime checks in the FFM-api, based on a system property (e.g. -Djava.lang.foreign.CHECK=true).

This would be similar to -Xcheck:jni.

Some checks we could possibly add are:
- An additional NULL check on the access memory segment when doing a memory access. This could help to make unsafe memory accesses (as a result of using a restricted method) a little bit safer.

The idea is that users can optionally turn on additional checks, that would normally be turned off for performance reasons.