RFR: 8279887: 2 Null pointer dereference defect groups in os_posix.cpp [v2]

[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

Harold Seigel has updated the pull request incrementally with one additional commit since the last revision:

  add comment

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7129/files
  - new: https://git.openjdk.java.net/jdk/pull/7129/files/fcde3737..8e79c91a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7129&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7129&range=00-01

  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 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