RFR: 8340438: RISC-V: minor improvement in base64

[Hamlin Li]

Hi,
Can you help to review this simple patch?
Thanks

Thanks @RealFYang for spotting this!

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/21105/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21105&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340438
  Stats: 6 lines in 1 file changed: 2 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21105.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21105/head:pull/21105

PR: https://git.openjdk.org/jdk/pull/21105