examples/demo/04-bytepusher: go back to showing help text on load failures.

If the VM was already running, and then a file failed to open at all (a
directory was dropped on the window, etc), this wouldn't go back to showing
the help text.

(cherry picked from commit 6c55fad411cbaa24ae4f488b4e349a21a3699b5c)

examples/demo/04-bytepusher/bytepusher.c

@@ -74,6 +74,8 @@ static bool load(BytePusher* vm, SDL_IOStream* stream, bool closeio) {
     size_t bytes_read = 0;
     bool ok = true;
 
+    vm->display_help = true;  // will set to false if load succeeds.
+
     SDL_memset(vm->ram, 0, RAM_SIZE);
 
     if (!stream) {