Debugger: Fix non-decimal display of negative numbers

Task-number: QTCREATORBUG-7322

Change-Id: Iabef05a8f9e77ce9ee3d6f7a456e876a0f823fd4
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>