... to prevent duplicating masters on slide copy-paste.
Also fixed a 10-year copy-paste error (pRefPage wasn't replaced
with pNPage).
Fixed argument evaluation order issue (aStream.GetEndOfData()
depends on Flush() but doesn't call it, so will return incorrect
result if called before aStream.GetBuffer()).
Replaced compare of hashes with results of stringify(),
because it removes useless overhead (hashes are calculated from
stringify() anyway, and are not cached anywhere).
Removed Flush() called from SvMemoryStream::GetBuffer(), because
it calls GetData(), which calls Flush() itself.

Thanks to Andras Timar for unit test framework.

Change-Id: Ia46d4e9a017fc628d424949a9d229045a249a4ca
Reviewed-on: https://gerrit.libreoffice.org/20367Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>