Only marked objects should be considered movable

Ruby's GC is incremental, meaning that during the mark phase (and also
the sweep phase) programs are allowed to run.  This means that programs
can allocate objects before the mark or sweep phase have actually
completed.  Those objects may not have had a chance to be marked, so we
can't know if they are movable or not. Something that references the
newly created object might have called the pinning function during the
mark phase, but since the mark phase hasn't run we can't know if there
is a "pinning" relationship.

To be conservative, we must only allow objects that are not pinned but
also marked to move.

gc.c

@@ -7599,7 +7599,7 @@ gc_is_moveable_obj(rb_objspace_t *objspace, VALUE obj)
                 return FALSE;
             }
         }
-        return !RVALUE_PINNED(obj);
+        return RVALUE_MARKED(obj) && !RVALUE_PINNED(obj);
 
       default:
         rb_bug("gc_is_moveable_obj: unreachable (%d)", (int)BUILTIN_TYPE(obj));