pull: Validate delta checksums more strongly
pull: Validate delta checksums more strongly

We need to check that it's 'ay'.  Also reuse the existing validation
function to check it's 32 bytes rather than potentially crashing with
assertion.

Just noticed this during a code review.

sujitfulse/ostree,gatispaeglis/ostree,sujitfulse/ostree,GNOME/ostree,GNOME/ostree,leandrosansilva/ostree,sujitfulse/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,sujitfulse/ostree,leandrosansilva/ostree,GNOME/ostree,gatispaeglis/ostree,sujitfulse/ostree,leandrosansilva/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,leandrosansilva/ostree,leandrosansilva/ostree