Fix infinite loop in MIME classinfo parser in ActiveQt

MIME extensions are parsed starting from the end by finding the last
extension separator ':' and chopping off the end of the string. Now
only 'extension - 1' chars are chopped which leaves the last extension
separator in place causing an infinite loop where the same separator
is found round after round.

Fixed by chopping the extension separator also.

Change-Id: I3df6008ad53dbecc17ac98647aedab63bf848963
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>