Fix off-by-one error in binary search

Fix off-by-one error in binary search

This one-line change makes the binary search in QTextEngine::findItem
behave consistently with the linear search that it replaced in commit
acf678e57ed088f3e56a551cac6c7c3322005750. The new behavior seems to
cause crashes in kword (and perhaps other applications) by triggering a
logClusters assert, although I have been unable to create a unit test
that reproduces this.

Task-number: QTBUG-17209
Done-by: Dr. Robert Marmorstein <robert@narnia.homeunix.com>
Change-Id: I68b79f024e9836e1cc8b0f3514889120541eb2ea
Reviewed-on: http://codereview.qt.nokia.com/2343
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>