coap_req: ignore already received blocks

[mniestroj]

coap_update_from_block() does not check previous value of block offset. Do this in golioth_coap_req_reply_handler(), by comparing what was set previously with the value updated from coap_update_from_block() (which is set based on BLOCK2 option of received CoAP packet).

This prevents processing same blocks multiple times.

Fixes: #408

[github-actions[bot]]

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