Check if first is not empty

If it's not checked that first is empty, crashes may happen.

Signed-off-by: Felix Weilbach <felix.weilbach@nextcloud.com>

src/libsync/discovery.cpp

@@ -177,7 +177,7 @@ void ProcessDirectoryJob::process()
         // For windows, the hidden state is also discovered within the vio
         // local stat function.
         // Recall file shall not be ignored (#4420)
-        bool isHidden = e.localEntry.isHidden || (f.first[0] == '.' && f.first != QLatin1String(".sys.admin#recall#"));
+        bool isHidden = e.localEntry.isHidden || (!f.first.isEmpty() && f.first[0] == '.' && f.first != QLatin1String(".sys.admin#recall#"));
 #ifdef Q_OS_WIN
         // exclude ".lnk" files as they are not essential, but, causing troubles when enabling the VFS due to QFileInfo::isDir() and other methods are freezing, which causes the ".lnk" files to start hydrating and freezing the app eventually.
         const bool isServerEntryWindowsShortcut = !e.localEntry.isValid() && e.serverEntry.isValid() && !e.serverEntry.isDirectory && FileSystem::isLnkFile(e.serverEntry.name);