Fix for "Mount-Points are purged from database on update/rescan."

NEWS

 ver 0.20.13 (not yet released)
+* database
+  - simple: don't purge mount points on update/rescan
 
 ver 0.20.12 (2017/11/25)
 * database

src/db/plugins/simple/Directory.cxx

@@ -109,7 +109,7 @@ Directory::PruneEmpty() noexcept
 	     child != end;) {
 		child->PruneEmpty();
 
-		if (child->IsEmpty())
+		if (child->IsEmpty() && !child->IsMount())
 			child = children.erase_and_dispose(child,
 							   DeleteDisposer());
 		else

src/db/update/Walk.cxx

@@ -104,7 +104,7 @@ inline void
 UpdateWalk::PurgeDeletedFromDirectory(Directory &directory)
 {
 	directory.ForEachChildSafe([&](Directory &child){
-			if (DirectoryExists(storage, child))
+			if (child.IsMount() || DirectoryExists(storage, child))
 				return;
 
 			editor.LockDeleteDirectory(&child);