Added more condition checks/cleanup to indexConvert

Now checking for out-of-order time/byte values and skipping them (positive- or
 negative-rate)

Fixed a bug in rate_from_dlna_index_filename(). It would get confused with
 normal-rate files that had a number as the last part of the basename.
 Added a check for "x". And indexConvert will bail out of the rate returned
 is "0.0".

Tested convert-dlna-scaled-helper and checked selected index files that
 had entries flagged by indexConvert.