test: input_decoder: remove assertion on pf_flush

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 cef875107b, 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).

test/src/input/decoder/input_decoder.c

@@ -96,8 +96,8 @@ static int DecoderDecode(decoder_t *dec, block_t *block)
 static void DecoderFlush(decoder_t *dec)
 {
     struct input_decoder_scenario *scenario = &input_decoder_scenarios[current_scenario];
-    assert(scenario->decoder_flush != NULL);
+    if (scenario->decoder_flush != NULL)
-    return scenario->decoder_flush(dec);
+        scenario->decoder_flush(dec);
 }
 
 static void CloseDecoder(vlc_object_t *obj)