Fixed assertion that broke SerialGC

Author: petermz

substratevm/src/com.oracle.svm.core.genscavenge/src/com/oracle/svm/core/genscavenge/GreyToBlackObjRefVisitor.java

@@ -99,7 +99,8 @@ public boolean visitObjectReferenceInline(Pointer objRef, int innerOffset, boole
                 counters.noteForwardedReferent();
                 // Update the reference to point to the forwarded Object.
                 Object obj = ohi.getForwardedObject(p, header);
-                assert ParallelGC.singleton().isInParallelPhase() || innerOffset < LayoutEncoding.getSizeFromObjectInGC(obj).rawValue();
+                assert ParallelGC.isEnabled() && ParallelGC.singleton().isInParallelPhase() ||
+                                innerOffset < LayoutEncoding.getSizeFromObjectInGC(obj).rawValue();
                 Object offsetObj = (innerOffset == 0) ? obj : Word.objectToUntrackedPointer(obj).add(innerOffset).toObject();
                 ReferenceAccess.singleton().writeObjectAt(objRef, offsetObj, compressed);
                 RememberedSet.get().dirtyCardIfNecessary(holderObject, obj);
@@ -112,7 +113,8 @@ public boolean visitObjectReferenceInline(Pointer objRef, int innerOffset, boole
             if (copy != obj) {
                 // ... update the reference to point to the copy, making the reference black.
                 counters.noteCopiedReferent();
-                assert ParallelGC.singleton().isInParallelPhase() || innerOffset < LayoutEncoding.getSizeFromObjectInGC(copy).rawValue();
+                assert ParallelGC.isEnabled() && ParallelGC.singleton().isInParallelPhase() ||
+                                innerOffset < LayoutEncoding.getSizeFromObjectInGC(copy).rawValue();
                 Object offsetCopy = (innerOffset == 0) ? copy : Word.objectToUntrackedPointer(copy).add(innerOffset).toObject();
                 ReferenceAccess.singleton().writeObjectAt(objRef, offsetCopy, compressed);
             } else {