mqtt_publish return value causes mqtt_error_str when not connected

[infn-ke]

Invoking mqtt_publish without being in a connected state gives a negative return value. Passing this return value into mqtt_error_str causes a SEGV.

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff756a35c in mqtt_error_str (error=-1094795586) at /usr/local/src/mqtt-c/src/mqtt.c:1773
1773            return MQTT_ERRORS_STR[offset];

Unfortunately MQTT_CLIENT_TRY_PACK is a macro so cannot get better backtrace without replacing it with a function.