RFR: 8268902: Testing for threadObj != NULL is unnecessary in handshake

[Coleen Phillimore]

The handshake code tests if the JavaThread->is_exiting or that the threadObj() is null. Ever since JDK-8244997, once the JavaThread is running, the _threadObj won't be null until JavaThread is destroyed. So testing is_exiting is all you need to do.
In gtest, the test JavaThread doesn't create a _threadObj JDK-8215948 so removing this unnecessary test allows writing gtests for handshakes.

Tested with tier1-6.

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

Commit messages:
 - 8268902: Testing for threadObj != NULL is unnecessary in handshake

Changes: https://git.openjdk.java.net/jdk/pull/4512/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4512&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268902
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4512.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4512/head:pull/4512

PR: https://git.openjdk.java.net/jdk/pull/4512