If an object isn't embedded it will have an ivptr

We don't need to check the existence if an ivptr because non-embedded
objects will always have one

gc.c

@@ -4077,9 +4077,8 @@ obj_memsize_of(VALUE obj, int use_all_types)
 
     switch (BUILTIN_TYPE(obj)) {
       case T_OBJECT:
-	if (!(RBASIC(obj)->flags & ROBJECT_EMBED) &&
-	    ROBJECT(obj)->as.heap.ivptr) {
-	    size += ROBJECT(obj)->as.heap.numiv * sizeof(VALUE);
+	if (!(RBASIC(obj)->flags & ROBJECT_EMBED)) {
+	    size += ROBJECT_NUMIV(obj) * sizeof(VALUE);
 	}
 	break;
       case T_MODULE: