SUBSTRING_INDEX UDF not working properly - patch

Hi,

When using the e-mail registration module combined with the sqlsrv module I ran into some problems with the 'SUBSTRING_INDEX' UDF. In the e-mail registration module it is used to check for existing users with the same user name. If one (or more) exist, an '_' is added + a serial number. So the first 'info@...' becomes 'info', the second 'info_1', the third 'info_2',...

The current SUBSTRING_INDEX function only works when the count parameter is set to a number greater than 0. If not it errors... The e-mail registration module uses -1 to look for the last occurrence.

Here is a patch for review and testing that should solve the problems with the SUBSTRING_INDEX function, fixing the e-mail registration module still needs some work!