Fix OOB write in fillExtraMM nonce handling

[MoneroOcean]

Motivation

• The fillExtraMM code trusted the TX_EXTRA_NONCE declared length byte and copied a 35-byte merged-mining tag without verifying the actual payload was present, leading to an out-of-bounds write and possible native heap corruption.
• The change aims to validate the nonce payload length before any rewrite/copy to prevent daemon-supplied malformed block templates from crashing or corrupting the process.

Description

• Add explicit bounds validation that extra_nonce_start and extra_nonce_start + extra_nonce_size are within miner_tx.extra before modifying the vector.
• Validate that the declared extra_nonce_size is at least MM_NONCE_SIZE and compute new_extra_nonce_size as an unsigned size_t.
• Update the nonce-length byte with a safe static_cast<uint8_t> and copy the merged-mining tag to a destination offset derived from the validated extra_nonce_start and new_extra_nonce_size, preserving original behavior for valid inputs.

Testing

• Reviewed the source patch and diff to confirm the added checks and adjusted offsets are correct.
• Displayed the modified region with nl -ba src/main.cc | sed -n '96,135p' to verify the new logic in context.
• No project build or automated test suite was run in this environment.

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Codex Task