`is_security_cookie` computes the last address in a terminal basic block by
iterating `bb.instruction_index` and indexing `ir.end_index - 1`. The BinExport2
protobuf spec omits `end_index` for single-element ranges, so protobuf returns 0
as the default. `0 - 1 = -1`, and -1 is not a key in `insn_address_by_index`,
raising `KeyError`.
Use `BinExport2Index.instruction_indices` to enumerate instruction indices, which
already handles single-instruction ranges via `HasField("end_index")`.
Willi Ballenthin
d1038e51f3