curl: MQTT CONNACK Packet Type Bypass leads to RCE via Malicious Broker

Summary: mqttverifyconnack in lib/mqtt.c never checks that the received packet type is actually a CONNACK 0x20. The constant MQTTMSGCONNACK is commented out at line 45, making the check impossible to write. A malicious broker can send any packet — e.g. PUBACK 0x40 — with remaininglength=2 and...