[foreign-abi] RFR: 8256466: MemoryLayout factories do not guard against null layouts [v2]

[Maurizio Cimadamore]

> This patch fixes the remaining issues with NPE in javadoc and implementation in the foreign-abi branch. For testing only few extra lines in the harness were required, the most difficult part being the VaList/Builder stuff (since these are scopd abstractions which can only be used in a certain way).

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Added checks for FunctionDescriptor

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Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/399/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/399/files/b36b4fa2..301d7a50

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=399&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=399&range=00-01

  Stats: 19 lines in 2 files changed: 13 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/399.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/399/head:pull/399

PR: https://git.openjdk.java.net/panama-foreign/pull/399