RFR: 8358066: Non-ascii package names gives compilation error "import requires canonical name"

[Archie Cobbs]

A simple counting bug in `Convert.utfNumChars()` causes bogus compiler errors for `import` statements of non-ASCII class names when the compiler is configured to use one of the older UTF-8 based `Name` table implementations (e.g., by specifying the `-XDuseUnsharedTable=true` flag).

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Commit messages:
 - Fix bug in Convert.utfNumChars().

Changes: https://git.openjdk.org/jdk/pull/25567/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25567&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358066
  Stats: 76 lines in 2 files changed: 71 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/25567.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25567/head:pull/25567

PR: https://git.openjdk.org/jdk/pull/25567