[foreign-memaccess+abi] RFR: 8294309: Downcall and Upcall unboxing code should reject heap segments [v3]

[Jorn Vernee]

> Recently I noticed that we don't check for heap segments anymore when unboxing memory segments to be passed to native code as addresses. This allows someone to pass a heap segment, which will be unboxed simply by calling `MemorySegment::address` which for heap segments returns the byte offset into the array. This is of course not a valid pointer.
> 
> This patch changes the unboxing logic to reject heap segments with an `IllegalArgumentException`.

Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  remove spurious import

-------------

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/737/files
  - new: https://git.openjdk.org/panama-foreign/pull/737/files/acc6c805..e811876c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=737&range=02
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=737&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/737.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/737/head:pull/737

PR: https://git.openjdk.org/panama-foreign/pull/737