RFR: 8242316: Shenandoah: Turn NULL-check into assert in SATB slow-path entry

[Roman Kennke]

In ShenandoahRuntime, we do an explicit NULL-check with return, that has
otherwise only an assert(false). We can turn that into a simple assert.

We might have left the check there as a safety-net, but it never
triggered anywhere. Also, we better make sure we never send NULL to the
SATB-slowpath to begin with.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8242316
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8242316/webrev.00/

Testing: hotspot_gc_shenandoah

Ok?

Roman