Fix crash in css rgb color handling.

Crashing since 2000 (or earlier).

Change-Id: I9f91d56f380be2421370b0acbee351461e1f0973
Reviewed-on: https://gerrit.libreoffice.org/8091
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>