RFR: 8269268: JDWP: Properly fix thread lookup assert in findThread()

[Chris Plummer]

Re-enable the assert that was disabled (with some overhead) by [JDK-8265683](https://bugs.openjdk.java.net/browse/JDK-8265683). Explanation is in the CR and also in comments included with the changes.

I tested by running `vmTestbase/nsk/jdb/suspend/suspend001/suspend001.java` and `vmTestbase/nsk/jdb/wherei/wherei001/wherei001.java` 100's of times, and did not see any failures. I also verified the original issue was still reproducible by temporarily not setting `gdata->handlingVMDeath = JNI_TRUE`, which did trigger the assert as expected.

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Commit messages:
 - Fix minor comment typo.
 - Better fix for dealing with tls related assert

Changes: https://git.openjdk.java.net/jdk/pull/4580/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4580&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269268
  Stats: 39 lines in 3 files changed: 32 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4580.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4580/head:pull/4580

PR: https://git.openjdk.java.net/jdk/pull/4580