message: [mdns] fix avahi size check
diff: @@ -517,9 +517,10 @@ otbrError PublisherAvahi::PublishService(uint16_t aPort, const char *aName, cons
  int rval;
  const char *value = va_arg(args, const char *);
  size_t valueLength = va_arg(args, size_t);
  - size_t needed = sizeof(AvahiStringList) + strlen(name) + valueLength;
  + // +1 for the size of "=", avahi doesn't need '\0' at the end of the entry
  + size_t needed = sizeof(AvahiStringList) - sizeof(AvahiStringList::text) + strlen(name) + valueLength + 1;
  - VerifyOrExit(used + needed < sizeof(buffer), errno = EMSGSIZE);
  + VerifyOrExit(used + needed <= sizeof(buffer), errno = EMSGSIZE);
  curr->next = last;
  last = curr;
  rval = sprintf(reinterpret_cast<char *>(curr->text), "%s=", name);
  