[foreign-memaccess+abi] RFR: 8266626: Check that the target address of a native call is not NULL

[Jorn Vernee]

This patch adds a check when unboxing the target address of a native call to check that the address is not `NULL`. Additionally, it adds checks earlier when an address is specified with a linking request, to check that that address is not `NULL`.

I didn't see any regressions on the CallOverhead* benchmarks.

Thanks,
Jorn

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Commit messages:
 - Add tests
 - WIP - Add NULL checks for target addresses (need tests)

Changes: https://git.openjdk.java.net/panama-foreign/pull/530/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=530&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266626
  Stats: 86 lines in 4 files changed: 79 ins; 4 del; 3 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/530.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/530/head:pull/530

PR: https://git.openjdk.java.net/panama-foreign/pull/530