RFR: Relax assert in SBS::is_safe()

[Roman Kennke]

With traversal I am hitting the assert in SBS::is_safe() (through 
weakref discovery) because GC got cancelled and the obj is not in 
to-space. It is not a problem with conc-mark because there we don't evac 
during marking.

The fix is to fall-through the in_cset() check when GC got cancelled, 
and check if there is an actual copy.

http://cr.openjdk.java.net/~rkennke/fixissafe/webrev.00/

Testing: hotspot_gc_shenandoah passes the occasional above failure now.

Good?

Roman