fix(advert): bounds-check optional fields before memcpy

[swaits]

Summary

AdvertDataParser::AdvertDataParser (src/helpers/AdvertDataHelpers.cpp) optimistically memcpy'd _lat / _lon / _extra1 / _extra2 from the supplied buffer before validating that the buffer was long enough. A crafted advert with all optional-field flag bits set and app_data_len = 1 over-read up to 12 bytes past the supplied slice. This change validates the length before each memcpy.

Background

The buggy pattern (verbatim, before this PR):

int i = 1;
if (_flags & ADV_LATLON_MASK) {
  memcpy(&_lat, &app_data[i], 4); i += 4;     // reads 8 bytes past app_data[1] regardless of app_data_len
  memcpy(&_lon, &app_data[i], 4); i += 4;
}
if (_flags & ADV_FEAT1_MASK) { memcpy(&_extra1, &app_data[i], 2); i += 2; }
if (_flags & ADV_FEAT2_MASK) { memcpy(&_extra2, &app_data[i], 2); i += 2; }
if (app_data_len >= i) {                       // length check happens AFTER the reads
  ...
  _valid = true;
}

The app_data argument in the live path is &pkt->payload[i] where pkt->payload is the 184-byte packet buffer (src/Packet.h), so OOB reads land inside the same allocation. That's the only thing keeping this from being a remote info-disclosure today. The fix prevents the OOB regardless of where the buffer happens to live, which is the right hygiene for a parser that takes a length parameter.

Change

src/helpers/AdvertDataHelpers.cpp — before each memcpy of an optional field, check that the requested byte count is available. If not, return early; _valid stays false, which is the same outcome callers already get for a too-short payload.

Why this is the minimal fix

The pattern matches the existing build-side AdvertDataBuilder::encodeTo reasoning. Rewriting the parser around a separate validator pass would add code without removing the underlying bug.

Risk / compatibility

• Wire format: unchanged.
• Behaviour: bit-for-bit identical for any valid advert (the lengths add up by construction). Only malformed adverts now stop earlier instead of writing junk into _lat / _lon / etc. and then being rejected by the trailing length check.

References

• CWE-125 — Out-of-bounds Read.
• CWE-1284 — Improper Validation of Specified Quantity in Input.