...with e.g. `--convert-to pdf abw/abi10006-1.abw` as obtained by
bin/get-bugzilla-attachments-by-mimetype, reporting

> vcl/source/fontsubset/cff.cxx:1697:63: runtime error: signed integer overflow: 55810 * 52845 cannot be represented in type 'int'

and

> vcl/source/fontsubset/cff.cxx:1137:38: runtime error: signed integer overflow: 233492066 * 52845 cannot be represented in type 'int'

Using unsigned here is faithful to the algorithm presented in Chapter 7
Encryption of "Adobe Type 1 Font Format" (<http://wwwimages.adobe.com/
www.adobe.com/content/dam/acom/en/devnet/font/pdfs/T1_SPEC.pdf>).

Change-Id: Ic27a5ebbe5f766fef425d241e14487ea4ae81da4
Reviewed-on: https://gerrit.libreoffice.org/73104
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>