Check ACK payload_len before memcpy, not after

[weebl2000]

Severity: Low

Summary

Both ACK packet handlers in Mesh::onRecvPacket copy 4 bytes from the payload via memcpy before checking whether payload_len >= 4:

1. Direct-route early ACK (line 83–86): memcpy then if (i <= pkt->payload_len)
2. Flood ACK (line 118–121): memcpy then if (i > pkt->payload_len)

When payload_len < 4, the memcpy reads stale data from within the 184-byte payload buffer. The result (ack_crc) is discarded in both cases — the subsequent length check prevents it from being used — but the ordering is incorrect.

Fix

Move the payload_len >= 4 check before the memcpy in both locations. Short ACK packets are now rejected without touching the payload data.

Test plan

• Normal ACK processing still works (direct and flood)
• Short/corrupt ACK packets are silently dropped
• Build tested on Heltec_v3_companion_radio_ble

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