input_decoder: ensure it is always flushed on vlc_input_decoder_Flush() call

Now that decoder_t::pf_flush is called right before every call to vlc_input_decoder_Delete, there are multiple cases without decoder_flush scenario entry and keeping it mandatory is not useful anymore.

Since cef87510, this was leading to spurrious test failure depending on whether the decoder thread exited before calling flush (no assertion) or if it could reach the call to pf_flush (assertion failure).

This MR then fix the failure to flush correctly because of that.