qmlplugindumper: do not pop up a window if an assert is triggered

On windows the debug version of qmlplugindumper used to show a panel on
assert failure. Failure should be silent.

Done-with: Thomas Hartmann <thomas.hartmann@digia.com>
Task-number: QTCREATORBUG-12644
Change-Id: I568f27e7d848aeafbffa059599f33b1b7ce0d2b9
Reviewed-by: Kai Koehne <kai.koehne@digia.com>