8156660: JDK-8150393 does not set _scan_in_progress properly

Reviewed-by: ehelin, mgerdin

hotspot/src/share/vm/gc/g1/g1ConcurrentMark.cpp

@@ -273,7 +273,7 @@ void G1CMRootRegions::prepare_for_scan() {
 
   // Currently, only survivors can be root regions.
   _claimed_survivor_index = 0;
-  _scan_in_progress = true;
+  _scan_in_progress = _survivors->regions()->is_nonempty();
   _should_abort = false;
 }