Stack buffer overflow in Zephyr Bluetooth Mesh solicitation handler

The Bluetooth Mesh On-Demand Private Proxy solicitation handler in subsys/bluetooth/mesh/solicitation.c copies a received Solicitation PDU into a fixed 17-byte stack buffer without bounding the source length. In sol_pdu_decrypt(), out is allocated as NET_BUF_SIMPLE(17) and then filled with net_buf_simple_add_mem(out, in->data, in->len); net_buf_simple_add() guards its tailroom only with __ASSERT_NO_MSG, which is compiled out in production builds, so when in->len > 17 the underlying memcpy writes attacker-controlled bytes past the 17-byte stack buffer. The copy occurs before any decryption or authentication, so no key material is required to trigger it. The oversized length arises because the mesh scan callback in subsys/bluetooth/mesh/adv.c calls net_buf_simple_restore() before dispatching to bt_mesh_sol_recv(), leaving buf->len covering the entire remaining advertising payload rather than just the Solicitation Service Data. After the parser locates the Service Data AD and consumes the Identification Type byte, the remaining buf->len is the 17-octet Network PDU plus any trailing advertising bytes, and prior to this fix there was no maximum-length check (only a minimum). An attacker can therefore append extra AD structures or padding after the Solicitation Service Data to make buf->len exceed 17. bt_mesh_scan_cb() is registered directly as the BLE scan callback, so buf is raw, unauthenticated advertising data received over the air. Any device in radio range can send a non-connectable advertisement carrying a crafted mesh Proxy Solicitation to a node that has CONFIG_BT_MESH_OD_PRIV_PROXY_SRV enabled and is currently eligible to be solicited (GATT proxy disabled, On-Demand Private Proxy enabled), with no pairing, bonding, or provisioning. The result is an attacker-controlled stack overwrite — plausibly leading to remote code execution and at minimum a reliable remote denial of service. The fix trims buf->len to the spec-fixed 17 octets (dropping the PDU if fewer remain) before decryption.