RFR: 8210235: JvmtiTrace::safe_get_current_thread_name is unsafe in debug builds

[Kim Barrett]

Please review this trivial change to avoid a possible assert, by
calling Thread::current_or_null rather than Thread::current in a
context where the NULL case is explicitly handled.

CR:
https://bugs.openjdk.java.net/browse/JDK-8210235

Webrev:
http://cr.openjdk.java.net/~kbarrett/8210235/open.00/

Testing: 
Locally (linux-64) verified change builds and ran some tests for
smoke.  I couldn't find any tests involving TraceJVMTI.