PVS-Studio warning V547 for NSS

struct SECItemStr {
  ...
  unsigned int len;
};

static SECStatus
pk11_mergeSecretKey(....)
{
  ...
  if (input.len < 0) {
    rv = SECFailure;
    goto done;
  }
  ...
}

This suspicious code was found in NSS project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression 'input.len < 0' is always false. Unsigned type value is never < 0. nss pk11merge.c 491

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