This bug caused crashes in the Windows build of sbbs when using the ;spy
sysop command with MQTT enabled.

Memory that is allocated in one DLL (e.g. sbbs.dll) must be freed in that same
DLL (e.g. sbbs.dll, not mosquitto.dll).

mosquitto_message_free() here frees the msg pointer, but we didn't allocate
it in mosquitto.dll in the first place, so that's wrong. The proper mosquitto
function to use in this case is mosquitto_message_free_contents().