Fix segfault on client sending malformed CONNACk.

CVE-xxxx-xxxx: If an authenticated client connected with MQTT v5 sent a
malformed CONNACK message to the broker a NULL pointer dereference occurred,
most likely resulting in a segfault. This will be updated with the CVE
number when it is assigned.
Affects versions 2.0.0 to 2.0.9 inclusive.

Closes #2163. Thanks to Bryan Pearson.