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: I0491ab745a546687772cd0d70fb6b8be0f03bb75
Reviewed-by: Kai Koehne <kai.koehne@digia.com>