umqtt.simple: Reject short CONNACK in connect.

[pablogventura]

Summary

• Fixes IndexError: bytes index out of range #1056: connect() used to assert on resp[0] / resp[1] after sock.read(4). A short or missing CONNACK (b"" or None on timeout) raised IndexError instead of an MQTT error.
• Raise MQTTException(-1) when CONNACK is missing, short, or not a 4-byte CONNACK. Broker return codes 1-5 are unchanged (MQTTException(resp[3])).

Test plan

• Unix micropython/umqtt.simple/test_umqtt_simple.py: valid CONNACK, empty read, and None read.

[pablogventura]

simple.mpy (mpy-cross, compiled as simple.py):

Version	bytes	vs master
master	2515	-
this commit	2531	+16

[pablogventura]

Follow-up: reuse a single raise MQTTException(r) for short/malformed CONNACK (r = -1) and broker codes. Same contract.

simple.mpy (mpy-cross, compiled as simple.py):

Version	bytes	vs master
master	2515	-
previous on this branch	2531	+16
this commit	2526	+11