Only override episode/season numbers if the file name has them

This fixes the issue in these forum threads:
http://forum.xbmc.org/showthread.php?tid=159206
http://forum.xbmc.org/showthread.php?tid=161064

where the combination of a date-based file name and a full nfo file would result in the episodes being added as "season -1, episode -1", causing all kinds of nonsense.

The code behind this was added in commit 2e5050a3c82e4c04401507f346f98ac423d1883b in order to fix ticket [#13665] (http://trac.xbmc.org/ticket/13665), and it's said in the ticket that a better solution to the problem would be to have the nfo file checked first to prevent the file duplication, but that's a little beyond my capabilities.  

This should be seen as a stopgap measure to repair the new bug (I'm thinking specifically in terms of v12.2) until a full fix can be made.