pre-empt null steps crash when uploading replays

this can only occur if ischartloaded() which counts [key] in songsbykey is true, but getstepsbykey which grabs [key] in stepsbykey returns a nullptr, which shoiuldn't be possible since songsbykey only has entries added after stepbykey has an entry added for the same key

in other words this shouldn't be possible, yet it consistently is crashing for me on hi19s eternus chart

src/DownloadManager.cpp

@@ -873,6 +873,8 @@ SetCURLPOSTScore(CURL*& curlHandle,
 	  curlHandle, form, lastPtr, "chartkey", hs->GetChartKey());
 	SetCURLFormPostField(curlHandle, form, lastPtr, "rate", hs->musics);
 	auto chart = SONGMAN->GetStepsByChartkey(hs->GetChartKey());
+	if (chart == nullptr)
+		return;
 	SetCURLFormPostField(curlHandle,
 						 form,
 						 lastPtr,
@@ -1062,6 +1064,8 @@ DownloadManager::UploadScoreWithReplayDataFromDisk(string sk,
 	if (offsets.size() > 0) {
 		replayString = "[";
 		auto steps = SONGMAN->GetStepsByChartkey(hs->GetChartKey());
+		if (steps == nullptr)
+			return;
 		vector<float> timestamps =
 		  steps->GetTimingData()->ConvertReplayNoteRowsToTimestamps(
 			hs->GetNoteRowVector(), hs->GetMusicRate());