fix: stop ctlwizchip() from dereferencing NULL arguments (AUD-005)

[occamsshavingkit]

Summary

Fixes unconditional NULL dereference in ctlwizchip() for documented no-argument commands.

Problem

Ethernet/wizchip_conf.c:436 unconditionally evaluated uint8_t tmp = *(uint8_t*) arg before dispatching to the command case:

int8_t ctlwizchip(ctlwizchip_type cwtype, void* arg) {
    uint8_t tmp = *(uint8_t*) arg;   // <-- dereferences before dispatch
    ...
    case CW_RESET_WIZCHIP:            // requires no argument
        wizchip_sw_reset();
        break;
    case CW_INIT_WIZCHIP:             // explicitly supports arg == NULL
        if (arg != 0) { ... }

CW_RESET_WIZCHIP requires no argument and CW_INIT_WIZCHIP explicitly supports arg == NULL for default socket buffers, so a documented no-argument call could dereference NULL.

Fix

• Remove the unconditional uint8_t tmp = *(uint8_t*) arg at the top of the function
• Move guarded dereferences inside only the cases that require arg:
  • CW_SYS_LOCK, CW_SYS_UNLOCK, CW_GET_SYSLOCK (W6100/W6300)
  • CW_GET_PHYPOWMODE, CW_GET_PHYLINK (W5100S/W5200/W5500/W6100/W6300)
• Each case returns -1 when arg == NULL
• CW_RESET_WIZCHIP and CW_INIT_WIZCHIP proceed without dereferencing

Verification

• Multi-chip compile check: passes for all 7 _WIZCHIP_ values
• ASan/UBSan: ctlwizchip(CW_RESET_WIZCHIP, NULL) and ctlwizchip(CW_INIT_WIZCHIP, NULL) do not fault; CW_SYS_LOCK with NULL returns -1