curl: MQTT state machine confusion: PINGRESP/DISCONNECT with non-zero remaining_length dispatches to stale nextstate

Summary: In lib/mqtt.c, the state machine in mqttdoing lines 894-911 in curl 8.20.0 does not validate that PINGRESP 0xD0 and DISCONNECT 0xE0 packets have remaininglength == 0 as required by MQTT 3.1.1 spec sections 3.13.1 and 3.14.1. A malicious broker can send a PINGRESP fixed header with non-ze...