Make sure that main thread is non yieldable

Main thread must be non yieldable even at "level 0" (bare API), outside
the 'pcall' from 'lua.c'.

lstate.c

@@ -395,6 +395,7 @@ LUA_API lua_State *lua_newstate (lua_Alloc f, void *ud) {
   g->allgc = obj2gco(L);  /* by now, only object is the main thread */
   L->next = NULL;
   g->Cstacklimit = L->nCcalls = LUAI_MAXCSTACK + CSTACKERR;
+  incnny(L);  /* main thread is always non yieldable */
   g->frealloc = f;
   g->ud = ud;
   g->warnf = NULL;