fix: validate recvfrom() return before indexing into buf (AUD-001)

[occamsshavingkit]

Summary

Fixes out-of-bounds write in loopback_udpc() where buf[ret] = 0x00 was executed before checking whether ret <= 0.

Problem

In Application/loopback/loopback.c:247-254, the UDP client loopback executed:

ret = recvfrom(sn, buf, size, destip, (uint16_t*)&destport);
buf[ret] = 0x00;                    // OOB if ret < 0 or ret == DATA_BUF_SIZE
printf("recv form[...]: %s\n", ...);
if (ret <= 0) { return ret; }       // check comes too late

• ret == DATA_BUF_SIZE writes one byte past the buffer (stack-buffer-overflow under ASan)
• Negative returns (e.g. SOCKERR_SOCKCLOSED = -13) index before the buffer (stack-buffer-underflow)

Fix

Move the ret <= 0 check before buf[ret] = 0x00 and printf(), preserving both after the guard.

Verification

• Multi-chip compile check: passes for all 7 _WIZCHIP_ values
• ASan/UBSan: host harness with mocked recvfrom() returning DATA_BUF_SIZE, -13, and 0 — no sanitizer abort