Do not clean "/" in pkgAcquire::Clean/pkgArchiveCleaner

Having "/" here is most likely a user configuration error and
may cause removal of import symlinks like /vmlinuz

Closes: #753531

apt-pkg/acquire.cc

@@ -486,6 +486,9 @@ bool pkgAcquire::Clean(string Dir)
    if (DirectoryExists(Dir) == false)
       return true;
 
+   if(Dir == "/")
+      return _error->Error(_("Clean of %s is not supported"), Dir.c_str());
+
    DIR *D = opendir(Dir.c_str());   
    if (D == 0)
       return _error->Errno("opendir",_("Unable to read %s"),Dir.c_str());

apt-pkg/clean.cc

@@ -34,7 +34,10 @@
 bool pkgArchiveCleaner::Go(std::string Dir,pkgCache &Cache)
 {
    bool CleanInstalled = _config->FindB("APT::Clean-Installed",true);
-      
+
+   if(Dir == "/")
+      return _error->Error(_("Clean of %s is not supported"), Dir.c_str());
+
    DIR *D = opendir(Dir.c_str());
    if (D == 0)
       return _error->Errno("opendir",_("Unable to read %s"),Dir.c_str());