Fixed bugs, which were found when running MQTT in non-blocking socket

This patch is for fixing bugs in MQTT that I found when MQTT_OVER_SSL is enabled.

getsockopt: Because getsockopt call clears the error, it should be called once when it is needed.
free: mbedtls_ssl_free has already released resources, but free is called again and it caused a fault.
fcntl: 3rd argument of fcntl is ignored when F_GETFL is used and put it as 0.

ambiot / ambz2_sdk