RFR: 8296007: crash in runtime/DefineClass/NullClassBytesTest.java

[Dean Long]

This is a simple fix to Java_NullClassBytesTest_nativeDefineClass so that it returns the jclass as expected.

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Commit messages:
 - return jclass

Changes: https://git.openjdk.org/jdk/pull/10892/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10892&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296007
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/10892.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10892/head:pull/10892

PR: https://git.openjdk.org/jdk/pull/10892