RUSTSEC-2026-0401 Out-of-bounds read in `train_from_buffer` when sample sizes overflow

ZDICT_trainFromBuffer is never told the length of the samples buffer, so train_from_buffer checked that the sample sizes add up to it. With overflow checks off, as in release builds, that sum wrapped around: sizes totalling 2^64 plus the buffer length passed the check, and the trainer read past the end of the buffer. zstd::dict::from_continuous relies on this check from zstd 0.4.10; older versions of zstd had their own, covered by a separate advisory.

This requires the sample sizes to come from an untrusted source. The flaw was corrected by summing the sizes with overflow checks.