Function panicback

Dependencies/lua/src/ltests.c:1244–1253

** does a long-jump back to "main program". */

** does a long-jump back to "main program".
*/
static int panicback (lua_State *L) {
  struct Aux *b;
  lua_checkstack(L, 1);  /* open space for 'Aux' struct */
  lua_getfield(L, LUA_REGISTRYINDEX, "_jmpbuf");  /* get 'Aux' struct */
  b = (struct Aux *)lua_touserdata(L, -1);
  lua_pop(L, 1);  /* remove 'Aux' struct */
  runC(b->L, L, b->paniccode);  /* run optional panic code */
  longjmp(b->jb, 1);
  return 1;  /* to avoid warnings */
}

static int checkpanic (lua_State *L) {
  struct Aux b;