[REVIEW-3-6 3-6-3] fix for rhbz#864858, content after sheet 256 is not imported from xlsx

[Markus Mohrhard]

Hey,

[1] prevents the data loss during import from xlsx. After sheet 256 no
content is imported any more because sheet index is considered
invalid.

Regards,
Markus

[1] http://cgit.freedesktop.org/libreoffice/core/commit/?id=458d68fffa883bc706638299e5a5b2d1399beb0d