FFI: Turn FFI finalizer table into a proper GC root.

Reported by Sergey Bronnikov. #1168

src/lj_state.c

@@ -196,6 +196,9 @@ static TValue *cpluaopen(lua_State *L, lua_CFunction dummy, void *ud)
   lj_lex_init(L);
   fixstring(lj_err_str(L, LJ_ERR_ERRMEM));  /* Preallocate memory error msg. */
   g->gc.threshold = 4*g->gc.total;
+#if LJ_HASFFI
+  lj_ctype_initfin(L);
+#endif
   lj_trace_initstate(g);
   lj_err_verify();
   return NULL;