golioth: fix zsock_setsockopt(..., TLS_HOSTNAME, ...) API use

[mniestroj]

zsock_setsockopt() expects to receive NULL terminated string and length of the string including NULL termination. Adjust code to follow that.

See Zephyr upstream commit 4234e801b544 ("net: sockets: Fix TLS_HOSTNAME option length inconsistency") for details.

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[mniestroj]

aaa91be5860fc66dc10aad6d36b9daa6b44e1325:9: WARNING:UNKNOWN_COMMIT_ID: Unknown commit id '4234e801b544', maybe rebased or not pulled?

is a false-positive in this context. This commit sha1 is from Zephyr upstream, not from this repo. It is explicitly stated so.