Fix an off by one error in the peephole optimizer when checking for unreachable code beyond a return.

Do a bounds check within find_op so it can return before going past the end as a safety measure.

https://github.com/python/cpython/commit/7db3c488335168993689ddae5914a28e16188447#diff-a33329ae6ae0bb295d742f0caf93c137
introduced this off by one error while fixing another one nearby.

This bug was shipped in all Python 3.6 and 3.7 releases.

The included unittest won't fail unless you do a clang msan build.