Editor: Fix infinite loop in BaseTextFind

Searching in a blockselection for the regular expressions
start of line '^' or line end '$' results in an infinite loop.

Only occures when the blockselection didn't contain the searched
regular expression.

Task-number: QTCREATORBUG-8159
Change-Id: I36412387ecce381300b75d0cd0a452ce5bf1094e
Reviewed-by: Eike Ziller <eike.ziller@digia.com>