bundle: fix offset calculation
bundle: fix offset calculation

The result of sizeof(sigsize) is unsigned, so -sizeof(sigsize) is wrong.
Convert it to signed first by storing it in offset (which is a goffset).

Signed-off-by: Jan Luebbe <9f5a27cdf1a1d6dd5e7372203be5c3a13524cbc8@debian.org>

rauc/rauc,rauc/rauc,ejoerns/rauc,jluebbe/rauc,ejoerns/rauc,rauc/rauc,jluebbe/rauc,jluebbe/rauc,ejoerns/rauc