And fix an issue discovered in the PPT parsing, where one of the
test files has dodgy values that trigger the assert in
BigInt::operator long().

Change-Id: Ic324ac38ecef4153cc434376317643ababe0a537
Reviewed-on: https://gerrit.libreoffice.org/47314Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>