[foreign-memaccess+abi] RFR: 8294621: Copying from a StringCharBuffer-backed MemorySegment results in crash

[Per Minborg]

This PR prevents `StringCharBuffer` arguments in `MemorySegment::ofBuffer`.

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Commit messages:
 - Prevent StringCharBuffer in MemorySegment::ofBuffer

Changes: https://git.openjdk.org/panama-foreign/pull/741/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=741&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294621
  Stats: 58 lines in 3 files changed: 58 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/741.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/741/head:pull/741

PR: https://git.openjdk.org/panama-foreign/pull/741