8276201: Shenandoah: Race results degenerated GC to enter wrong entry…

… point

Reviewed-by: shade
Backport-of: dbf5100dd705fbe4a3aeae49405ca541d581f106

src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp

@@ -1735,7 +1735,9 @@ void ShenandoahHeap::op_degenerated(ShenandoahDegenPoint point) {
       }
 
     case _degenerated_mark:
-      op_final_mark();
+      if (is_concurrent_mark_in_progress()) {
+        op_final_mark();
+      }
       if (cancelled_gc()) {
         op_degenerated_fail();
         return;