RFR (XS): 8217765: Internal Error (javaCalls.cpp:61) guarantee(thread->can_call_java()) failed

[David Holmes]

Bug: https://bugs.openjdk.java.net/browse/JDK-8217765
webrev: http://cr.openjdk.java.net/~dholmes/8217765/webrev.13/

A JDK 11 user hit what should be an "impossible" code path in the new 
nestmate logic, whereby a compiler thread attempted to load a class and 
failed because it can't execute Java code. We have not been able to get 
enough information to determine exactly how this arises so I have simply 
put together a defensive fix that skips the attempted load if in a 
thread that can't run Java - the same technique that is already used in 
another part of the nestmate logic.

There's no regression test as I cannot determine a way to make this code 
path execute. But the submitter has tested it with their application and 
their problem has been resolved.

Other testing: mach5 tiers 1-3

Thanks,
David