Roots backend fails to update when invalid data encountered in mtime_map

[kiorky]

Traceback (most recent call last):
  File "/srv/salt/makina-states/src/salt/salt/daemons/masterapi.py", line 211, in fileserver_update
    fileserver.update()
  File "/srv/salt/makina-states/src/salt/salt/fileserver/__init__.py", line 282, in update
    self.servers[fstr]()
  File "/srv/salt/makina-states/src/salt/salt/fileserver/roots.py", line 124, in update
    file_path, mtime = line.split(':', 1)
ValueError: need more than 1 value to unpack

[terminalmage]

I've renamed this issue so that it describes what the actual issue is.