Fix memory leak of unhandled certificateError

If a user does not handle onCertificateError the
instance of QQuickWebEngineCertificateError never
gets to JavaScript land and never gets deleted.

Create a strong reference before emitting the
onCertificateError to guard against the memory
leak.

Change-Id: I49dbf89445d32291e2f52976f0f5e9deda201fcb
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>