[foreign-memaccess+abi] RFR: Fix arithmetic bug in SegmentAllocator

[Per Minborg]

This PR proposes to fix an arithmetic bug that can manifest itself for large strings and multi-byte encoding.

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Commit messages:
 - Fix arithmetic bug

Changes: https://git.openjdk.org/panama-foreign/pull/903/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=903&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/903.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/903/head:pull/903

PR: https://git.openjdk.org/panama-foreign/pull/903