MessageImpl::errstr() can attempt to construct std::string with NULL

[ephisino]

Bug in MessageImpl::errstr() can attempt to construct std::string with NULL which is defined as undefined behavior by the standard.

Description

recently introduced code in MessageImpl::errstr() can try to construct a std::string with a null ptr

const char *rd_kafka_message_errstr (const rd_kafka_message_t *rkmessage) {
        if (!rkmessage->err)
                return NULL;

  std::string         errstr() const {
    /* message_errstr() is only available for the consumer. */
    if (rk_type_ == RD_KAFKA_CONSUMER)
      return std::string(rd_kafka_message_errstr(rkmessage_));

How to reproduce

call errstr from a consumer on message with no error

IMPORTANT: Always try to reproduce the issue on the latest released version (see https://github.com/edenhill/librdkafka/releases), if it can't be reproduced on the latest version the issue has been fixed.

Checklist

IMPORTANT: We will close issues where the checklist has not been completed.

Please provide the following information:

• librdkafka version (release number or git tag): v1.5.2
• Apache Kafka version: <REPLACE with e.g., 0.10.2.3>
• librdkafka client configuration: <REPLACE with e.g., message.timeout.ms=123, auto.reset.offset=earliest, ..>
• Operating system: Linux x86-64
• Provide logs (with debug=.. as necessary) from librdkafka
• Provide broker log excerpts
• Critical issue