[foreign-memaccess] RFR: JDK-8243089: Foreign address manipulation always fails with exception

[Maurizio Cimadamore]

This patch fixes a silly mistake in Foreign implementation. Instead of checking whether `MemoryAddress::segment`
returns null, we instead check if it returns `AbstractMemorySegmentImpl::NOTHING`.

I've also added positive test, to make sure we catch these issues from now on.

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Commit messages:
 - Fix behavior of Foreign::withSize/Foreign::asMallocSegment

Changes: https://git.openjdk.java.net/panama-foreign/pull/118/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/118/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-JDK-8243089
  Stats: 28 lines in 3 files changed: 26 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/118.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/118/head:pull/118

PR: https://git.openjdk.java.net/panama-foreign/pull/118