[foreign-jextract] RFR: Fix Cstring not to call withSize on already bound addresses

[Maurizio Cimadamore]

There was a spurious call on withSize left on the synthetic Cstring type which did not make sure that the input address
was indeed unchecked.

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Commit messages:
 - Fix Cstring not to call withSize on already bound addresses

Changes: https://git.openjdk.java.net/panama-foreign/pull/121/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/121/webrev.00
  Stats: 6 lines in 2 files changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/121.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/121/head:pull/121

PR: https://git.openjdk.java.net/panama-foreign/pull/121