Fix issue of possible dead reference to Options

Drive and Options are both Singlton, their destruct sequence is
random. To avoid the dead reference, Drive destructor should not
reference to Options.

src/filesystem/Drive.cpp

@@ -117,6 +117,8 @@ Drive::Drive()
     : m_mountable(true),
       m_cleanup(false),
       m_connect(false),
+      m_diskCacheFolder(
+          QS::Configure::Options::Instance().GetDiskCacheDirectory()),
       m_client(ClientFactory::Instance().MakeClient()),
       m_transferManager(
           TransferManagerFactory::Create(TransferManagerConfigure())) {
@@ -150,11 +152,9 @@ void Drive::CleanUp() {
   if (!GetCleanup()) {
     // remove disk cache folder if existing
     // log off, to avoid dead reference to log (a singleton)
-    string diskfolder =
-        QS::Configure::Options::Instance().GetDiskCacheDirectory();
-    if (QS::Utils::FileExists(diskfolder) &&
-            QS::Utils::IsDirectory(diskfolder).first) {
-      DeleteFilesInDirectory(diskfolder, true);  // delete folder itself
+    if (QS::Utils::FileExists(m_diskCacheFolder) &&
+        QS::Utils::IsDirectory(m_diskCacheFolder).first) {
+      DeleteFilesInDirectory(m_diskCacheFolder, true);  // delete folder itself
     }
 
     m_client.reset();

src/filesystem/Drive.h

@@ -285,6 +285,8 @@ class Drive : public Singleton<Drive> {
 
   bool m_connect;  // denote if drive connected to storage
 
+  std::string m_diskCacheFolder;
+
   boost::shared_ptr<QS::Client::Client> m_client;
   boost::shared_ptr<QS::Client::TransferManager> m_transferManager;
   boost::shared_ptr<QS::Data::Cache> m_cache;