util: Fix segfault when doing a backup with an empty backup_path

Revert change of 'util: Fix occurances where back() might be called on an
empty string' for `zenfs_tool_backup()`.

Otherwise test/utils/0040_backup.sh segfaults when building ZenFS with
RocksDB 6.29.fb (c++11).

Signed-off-by: Dennis Maisenbacher <dennis.maisenbacher@wdc.com>
Signed-off-by: Hans Holmberg <hans.holmberg@wdc.com>

util/zenfs.cc

@@ -520,7 +520,7 @@ int zenfs_tool_backup() {
     }
 
     std::string backup_path = FLAGS_backup_path;
-    AddDirSeparatorAtEnd(backup_path);
+    if (backup_path.size() > 0 && backup_path.back() != '/') backup_path += "/";
     io_status = zenfs_tool_copy_dir(zenFS.get(), backup_path,
                                     FileSystem::Default().get(), FLAGS_path);
   }