RFR: 8279887: 2 Null pointer dereference defect groups in os_posix.cpp

[Harold Seigel]

Please review this small fix to prevent possible Null pointer dereferences.  The fix adds a Null check to prevent trying to park a Null thread.  The Null check is needed because UNSAFE_ENTRY calls thread_from_jni_environment(), which returns NULL if the env thread is terminated. 

The fix was tested with Mach5 tiers 1-2 on Linux, Mac OS, and Windows, and Mach5 tiers 3-5 on Linux x64.

Thanks, Harold

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Commit messages:
 - 8279887: 2 Null pointer dereference defect groups in os_posix.cpp

Changes: https://git.openjdk.java.net/jdk/pull/7129/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7129&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279887
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7129.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7129/head:pull/7129

PR: https://git.openjdk.java.net/jdk/pull/7129