Fix OpenVPN client: stack corruption, memory leak, key zeroing, dangling netif

[Copilot]

Code review of the OpenVPN client implementation surfaced several bugs across the binding, adapter, and core layers.

Fixes

• Lua stack corruption (luat_lib_netdrv.c): Extra lua_pop(L, 1) pops the config table before luat_netdrv_setup() consumes it — removed
• Memory leak on register failure (luat_netdrv_openvpn.c): TLS contexts and cert buffers from ovpn_client_init() leaked when luat_netdrv_register() failed — added ovpn_client_stop() before free
• Private key not wiped (luat_netdrv_openvpn_client.c): client_key_buf freed without zeroing — now uses mbedtls_platform_zeroize() to prevent key material lingering in heap
• Dangling netif pointer (luat_netdrv_openvpn_client.c): net_lwip2_set_netif() not cleared on stop, leaving stale pointer in lwIP adapter table — added net_lwip2_set_netif(adapter_index, NULL)
• Spurious disconnect event: OVPN_EVENT_DISCONNECTED fired even when client was never started — now gated on was_started
• Dead code: Removed unused openvpn_shutdown() (defined but never wired to any netdrv callback)

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud