more check for null pointer before deref
more check for null pointer before deref

in:
mqtt_tcpclient_recv()
mqtt_tcpclient_sent_cb()
mqtt_tcpclient_discon_cb()
mqtt_tcpclient_recon_cb()

nabovarme/MeterLogger,nabovarme/MeterLogger,nabovarme/MeterLogger,nabovarme/MeterLogger,nabovarme/MeterLogger