RFR: 8292158: AES-CTR cipher state corruption with AVX-512

[AJ Ferguson]

Fix cipher state corruption when encrypting/decrypting less than 16 bytes at a time.

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Commit messages:
 - exit preloop if len_reg is 0

Changes: https://git.openjdk.org/jdk/pull/9635/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9635&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292158
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9635.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9635/head:pull/9635

PR: https://git.openjdk.org/jdk/pull/9635