[foreign-memaccess+abi] RFR: 8266626: Check that the target address of a native call is not NULL [v2]

[Jorn Vernee]

> This patch adds a check when unboxing the target address of a native call to check that the address is not `NULL`. Additionally, it adds checks earlier when an address is specified with a linking request, to check that that address is not `NULL`.
> 
> I didn't see any regressions on the CallOverhead* benchmarks.
> 
> Thanks,
> Jorn

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

  - Merge check symbol code
  - add missing documentation

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

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/530/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/530/files/8bc89f32..4e871492

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=530&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=530&range=00-01

  Stats: 36 lines in 6 files changed: 19 ins; 12 del; 5 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/530.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/530/head:pull/530

PR: https://git.openjdk.java.net/panama-foreign/pull/530