RFR: 8320618: NPE: Cannot invoke "java.lang.constant.ClassDesc.isArray()" because "this.sym" is null

[Adam Sotona]

ClassFile API StackMapGenerator caused a NPE for a test case with invalid sequence of instructions.
This fix simply avoids the NPE and adds StackMapsTest::testInvalidAALOADStack

Please review.

Thanks,
Adam

-------------

Commit messages:
 - 8320618: NPE: Cannot invoke "java.lang.constant.ClassDesc.isArray()" because "this.sym" is null

Changes: https://git.openjdk.org/jdk/pull/16793/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16793&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320618
  Stats: 15 lines in 2 files changed: 12 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16793.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16793/head:pull/16793

PR: https://git.openjdk.org/jdk/pull/16793