RFR (xs) 8151728: jni test crashes JVM assert(_handle != __null) failed: resolving NULL handle

[Coleen Phillimore]

Summary: New assert needs to check for null.

If there's an OOM during Throwable.backtrace creation, the backtrace can 
be null, so the BacktraceIterator() checks for null, but the assert I 
added didn't.

Retested on failing test, Throwable tests and instrumented VM to force 
this (removed instrumentation).

open webrev at http://cr.openjdk.java.net/~coleenp/8151728/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8151728

Thanks,
Coleen