FFI: Don't assert on #1LL (5.2 compatibility mode only).

Reported by Denis Golovan.

src/lj_carith.c

@@ -272,6 +272,15 @@ int lj_carith_op(lua_State *L, MMS mm)
   return lj_carith_meta(L, cts, &ca, mm);
 }
 
+/* No built-in functionality for length of cdata. */
+int lj_carith_len(lua_State *L)
+{
+  CTState *cts = ctype_cts(L);
+  CDArith ca;
+  carith_checkarg(L, cts, &ca);
+  return lj_carith_meta(L, cts, &ca, MM_len);
+}
+
 /* -- 64 bit integer arithmetic helpers ----------------------------------- */
 
 #if LJ_32 && LJ_HASJIT