RFR: 8280422: thread_from_jni_environment can never return NULL

[David Holmes]

Analysing the code and its use (see JBS issue for gory details) we can see that `thread_from_jni_environment` can never actually return NULL, so we change it to not appear to do so and thus keep static analysis tools happy.  We also always validate the incoming JNIEnv for debug builds and scrap the unused `VerifyJNIEnvThread` flag.

Testing: tiers 1-5

Thanks,
David

-------------

Commit messages:
 - 8280422: thread_from_jni_environment can never return NULL

Changes: https://git.openjdk.java.net/jdk/pull/7193/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7193&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280422
  Stats: 24 lines in 3 files changed: 10 ins; 4 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7193.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7193/head:pull/7193

PR: https://git.openjdk.java.net/jdk/pull/7193