RFR (XS): 8242241: add assert to ClassUnloadEventImpl::className

[serguei.spit...@oracle.com]

Please, review a fix for minor JDI enhancement:
  https://bugs.openjdk.java.net/browse/JDK-8242241

Webrev:
  http://cr.openjdk.java.net/~sspitsyn/webrevs/2020/8242241-jdi-add-assert.1/

Summary:
  A useful assert is introduced to check the class signature format is correct.

Testing:
  The mach5 tier5 results are good.

Thanks,
Serguei