PVS-Studio warning V666 for OpenSSL

static int asn1_cb(const char *elem, int len, void *bitstr)
{
  ....
  if (!strncmp(vstart, "ASCII", 5))
    arg->format = ASN1_GEN_FORMAT_ASCII;
  else if (!strncmp(vstart, "UTF8", 4))
    arg->format = ASN1_GEN_FORMAT_UTF8;
  else if (!strncmp(vstart, "HEX", 3))
    arg->format = ASN1_GEN_FORMAT_HEX;
  else if (!strncmp(vstart, "BITLIST", 3))
    arg->format = ASN1_GEN_FORMAT_BITLIST;
  else
  ....
}

This suspicious code was found in OpenSSL project by PVS-Studio static code analyzer.
Warning message is:
V666 Consider inspecting third argument of the function 'strncmp'. It is possible that the value does not correspond with the length of a string which was passed with the second argument. asn1_gen.c 371

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