rust/bupsplit: minor idiomatic fixes
rust/bupsplit: minor idiomatic fixes

This fixes a few un-idiomatic bits in Rust bupsplit code, getting rid
of some unchecked casts and an assert statement.

Closes: #1502
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree