Fix the size of run-time generated root classes.

The size of the new class with should be sizeof(Class), not sizeof(struct objc_class).

Test/AllocatePair.m

@@ -15,6 +15,8 @@ int main()
 {
 	Class a, b, c, d, e;
 
+	assert(class_getInstanceSize(objc_allocateClassPair(Nil, "Empty", 0)) == sizeof(Class));
+	a = objc_allocateClassPair([Test class], "A", 0);
 	a = objc_allocateClassPair([Test class], "A", 0);
 	objc_registerClassPair(a);
 

runtime.c

@@ -726,7 +726,7 @@ Class objc_allocateClassPair(Class superclass, const char *name, size_t extraByt
 
 	if (Nil == superclass)
 	{
-		newClass->instance_size = sizeof(struct objc_class);
+		newClass->instance_size = sizeof(struct objc_class*);
 	}
 	else
 	{