libtracker-miner: Don't crash in _file_system_get_file() if node non-NULL
libtracker-miner: Don't crash in _file_system_get_file() if node non-NULL

This occurred where parent_node was unset because of strange URI themes, e.g.
sftp://.

Fixes GB#672308.

hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker