db/update/Archive: validate directory names

Fixes assertion failure if the ZIP file contains a path that begins
with a slash.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1793

src/db/update/Archive.cxx

@@ -50,6 +50,9 @@ UpdateWalk::UpdateArchiveTree(ArchiveFile &archive, Directory &directory,
 {
 	const auto [child_name, rest] = Split(name, '/');
 	if (rest.data() != nullptr) {
+		if (!IsAcceptableFilename(child_name))
+			return;
+
 		//add dir is not there already
 		Directory *subdir = LockMakeChild(directory, child_name);
 		subdir->device = DEVICE_INARCHIVE;