libtracker-miner: Fix possible warnings
This might be called in the right circumstances that the current_dir is NULL, cater for that. Fixes possible warnings like:
tracker-miner-f[2965]: g_file_equal: assertion 'G_IS_FILE (file1)' failed tracker-miner-f[2965]: g_file_has_prefix: assertion 'G_IS_FILE (file)' failed