Editor: Fix replace all with regular expression.

Change the accepted length of a search result, if the replacing text
is longer then 0 character. If it would accept also for 0 length
character we would have an endlles loop

Task-Number: QTCREATORBUG-8317
Change-Id: I81a6dccb2a1e16faa6ce8e834ea4b63bae84ecfa
Reviewed-by: Eike Ziller <eike.ziller@digia.com>