Fixed debsign_process when processing signed dsc.

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>
Added processing invalid signed data.

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>
Fixed verification of checksums.

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>