Add missing address operator in cm_polylib.c

Also avoid nullptr dereference.

code/qcommon/cm_polylib.c

@@ -276,7 +276,7 @@ winding_t	*CopyWinding (winding_t *w)
 	winding_t	*c;
 
 	c = AllocWinding (w->numpoints);
-	size = (intptr_t) ((winding_t *)0)->p[w->numpoints];
+	size = (intptr_t)&(w->p[w->numpoints]) - (intptr_t)w;
 	Com_Memcpy (c, w, size);
 	return c;
 }