RFR: 8265683: vmTestbase/nsk/jdb tests failed with "JDWP exit error AGENT_ERROR_INTERNAL(181)"

[Chris Plummer]

This bug was introduced by my recent changes for [JDK-8265028](https://bugs.openjdk.java.net/browse/JDK-8265028), which attempted to speed up ThreadNode lookups by not looking in the runningThreads list if the TLS lookup failed. At the time it was thought that the thread could not possibly be on the list, but it turns out sometimes it can.

For now I'm just doing a quick fix to replace the assert being triggered with a lookup instead, which is pretty much how it worked before JDK-8265028. However, I eventually want to get back to not having to do the lookup, but first I need to better understand why this is happening in the first place, and the tests are failing too often to wait for that, thus the quick fix.

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Commit messages:
 - Lookup node in runningThreads list rather than assert.

Changes: https://git.openjdk.java.net/jdk/pull/3634/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3634&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265683
  Stats: 9 lines in 1 file changed: 7 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3634.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3634/head:pull/3634

PR: https://git.openjdk.java.net/jdk/pull/3634