RFR: 8268418: [lworld] VM assert illegal mirror klass when calling Class::getInterfaces on a flat array

[Mandy Chung]

Fix the assert in `JVM_GetClassInterfaces` to check for flat array.

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Commit messages:
 - 8268418: [lworld] VM assert illegal mirror klass when calling Class::getInterfaces on a flat array

Changes: https://git.openjdk.java.net/valhalla/pull/440/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=440&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268418
  Stats: 33 lines in 2 files changed: 32 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/440.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/440/head:pull/440

PR: https://git.openjdk.java.net/valhalla/pull/440