C++: fix invalid common prefix calculation.

This was the indirect cause for "random" freezes when using
code-completion. The direct cause was a while loop counting down to 0,
but starting with a negative value. Both are fixed now.

Task-number: QTCREATORBUG-8472
Task-number: QTCREATORBUG-8532
Change-Id: I782a46be3ac8282d7484d0bf0c6fd30be6b171c8
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>