[foreign-memaccess+abi] RFR: 8268633: CLinker::toJavaString doesn't check for NULL address

[Maurizio Cimadamore]

As the subject says, a couple of function in CLinker do not check for NULL addresses passed as arguments.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.java.net/panama-foreign/pull/559/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=559&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268633
  Stats: 154 lines in 4 files changed: 88 ins; 61 del; 5 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/559.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/559/head:pull/559

PR: https://git.openjdk.java.net/panama-foreign/pull/559