Fix possible crash in CDN file loading.

Telegram/SourceFiles/storage/download_manager_mtproto.cpp

@@ -487,6 +487,13 @@ void DownloadMtprotoTask::removeSession(int sessionIndex) {
 			redirect.push_back({ requestId, requestData.offset });
 		}
 	}
+	for (auto &[requestData, bytes] : _cdnUncheckedParts) {
+		if (requestData.sessionIndex == sessionIndex) {
+			const auto newIndex = _owner->chooseSessionIndex(dcId());
+			Assert(newIndex < sessionIndex);
+			requestData.sessionIndex = newIndex;
+		}
+	}
 	for (const auto &[requestId, offset] : redirect) {
 		cancelRequest(requestId);
 		const auto newIndex = _owner->chooseSessionIndex(dcId());

Telegram/SourceFiles/storage/download_manager_mtproto.h

@@ -163,7 +163,7 @@ protected:
 private:
 	struct RequestData {
 		int offset = 0;
-		int sessionIndex = 0;
+		mutable int sessionIndex = 0;
 		int requestedInSession = 0;
 		crl::time sent = 0;