RFR: 8282852: Debug agent asserts in classTrack_addPreparedClass()

[Chris Plummer]

Fix incorrect assert that assumed the debug agent should never see a ClassPrepare event for a class that it already saw when calling GetLoadedClasses. Details can be found in the bug description.

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Commit messages:
 - Fix 8282852. No longer assert just because class has been seen before.

Changes: https://git.openjdk.java.net/jdk/pull/7752/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7752&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282852
  Stats: 19 lines in 1 file changed: 12 ins; 5 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7752.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7752/head:pull/7752

PR: https://git.openjdk.java.net/jdk/pull/7752