[foreign-abi] RFR: 8240173: Confusing overflow error when trying to dereference a nothing segment

[Maurizio Cimadamore]

This simple patch attempts to generate a more explicit error message when trying to dereference an address which is based on the Nothing segment. Note that the first problem here is caused by the fact that the Nothing segment is considered "small" - which means most of the real address values will fall outside its range. While we could simply fix that (which will improve the error message - from reporting an overflow during offset computation to report an access outside the bounds of the segment), I thought it was better to mark the Nothing segment as non-accessible, and generate an explicit error message.

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Commits:
 - d3ea2ea0: Clarify exception message when clients attempt to dereference the Nothing segment.

Changes: https://git.openjdk.java.net/panama-foreign/pull/34/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/34/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8240173
  Stats: 30 lines in 3 files changed: 21 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/34.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/34/head:pull/34

PR: https://git.openjdk.java.net/panama-foreign/pull/34