Windows: Silence warnings about BitBlt() failing.

After showing the lock screen, spurious warnings about BitBlt()
failing occur with last error set to ERROR_SUCCESS or
ERROR_INVALID_HANDLE.

Task-number: QTBUG-35926
Task-number: QTBUG-29716

Change-Id: I3e899cdf1ae60e76d13b81caec43716ad7c9725d
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>