It turned out in https://github.com/eclipse/paho.mqtt.c/issues/385 that
Socket_getReadySocket() could remove a list element, which is currently
in use by a function called by Socket_noPendingWirte().
And this leads to memory corruption or segmentation fault.
To fix this, this call of Socket_PendingWrites() in MQTTAsync.c needs to be protected by mutex_socket.
Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>