Details (mostly comments)

lgc.c

@@ -1565,6 +1565,7 @@ static void incstep (lua_State *L, global_State *g) {
 */
 void luaC_step (lua_State *L) {
   global_State *g = G(L);
+  lua_assert(!g->gcemergency);
   if (g->gcrunning) {  /* running? */
     if(isdecGCmodegen(g))
       genstep(L, g);