Add more precondition checks when handle async operation result

PR  #19460.

src/base/bittorrent/torrentimpl.cpp

@@ -1598,6 +1598,7 @@ void TorrentImpl::applyFirstLastPiecePriority(const bool enabled)
 
 void TorrentImpl::fileSearchFinished(const Path &savePath, const PathList &fileNames)
 {
+    if (m_maintenanceJob == MaintenanceJob::HandleMetadata)
         endReceivedMetadataHandling(savePath, fileNames);
 }
 
@@ -1630,7 +1631,13 @@ std::shared_ptr<const libtorrent::torrent_info> TorrentImpl::nativeTorrentInfo()
 
 void TorrentImpl::endReceivedMetadataHandling(const Path &savePath, const PathList &fileNames)
 {
+    Q_ASSERT(m_maintenanceJob == MaintenanceJob::HandleMetadata);
+    if (m_maintenanceJob != MaintenanceJob::HandleMetadata) [[unlikely]]
+        return;
+
     Q_ASSERT(m_filePaths.isEmpty());
+    if (!m_filePaths.isEmpty()) [[unlikely]]
+        m_filePaths.clear();
 
     lt::add_torrent_params &p = m_ltAddTorrentParams;