fuzz: use std::optional for sep_pos variable

Summary:

This PR changes the original size_t sep_pos to std::optional<size_t>
sep_post_opt to remove the warning when compiling fuzz tests.

Backport of core PR18901.

Note that we don't have the warning for some reason.

Test Plan:

ninja bitcoin-fuzzers
./src/test/fuzz/asmap_direct <path_to_corpus>

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9060