RFR (XXS): 8026376: assert(false) failed: DEBUG MESSAGE: exception pc already set

[Christian Thalinger]

https://bugs.openjdk.java.net/browse/JDK-8026376
http://cr.openjdk.java.net/~twisti/8026376/webrev.00/

8026376: assert(false) failed: DEBUG MESSAGE: exception pc already set
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It made me suspicious that only the second assert fired (the exception pc check) and not the first one (the exception oop check). I checked all the places where JavaThread::exception_oop_offset() was used to clear the exception oop to see if we also clear the exception pc. 

In the deopt blob we have such a case. On x86 we clear both but on SPARC we don't. 

Added the missing null-store instruction makes the test case pass.

Reading the exception pc is not really required but I've added it for completeness.