Prevent segvs in JNI.

A segv in JNI code (without CheckJNI) is hard to debug as we fail to see
stacks, pending exceptions.. Make JNI code robust to null arguments, but don't
go to the lengths that CheckJNI does in also sanity checking arguments, the
priority here is just to stop the VM from crashing.

Bug 10305723

Also, allow an unknown throw location not to crash exception dumping.

Found/useful for debugging bug 10331039.

Change-Id: Ib1e5540df87e2d9ce7570802d624baae7797611f