correctly look for /Root in the last trailer section
A mistake snuck into commit 76e546ce, taking the last element of the xrefs array as the "last" trailer section. But the array is filled in reverse order by following the chain of startxref and /Prev pointers, so the (logical) last/latest section is xrefs[0].
Based on !47 (closed) (parse_xrefs), so merge that first.