Add length checks in from_bytes() for ATT and SMP protocol parsers
to prevent IndexError crashes from empty PDUs sent by remote Bluetooth
devices. Also add buffer size limit and UTF-8 error handling in HFP
protocol to prevent memory exhaustion and decode crashes.
- bumble/att.py: validate PDU is non-empty before accessing pdu[0]
- bumble/smp.py: validate PDU is non-empty before accessing pdu[0]
- bumble/hfp.py: limit buffer to 64KB, handle invalid UTF-8 gracefully
These issues can be triggered by a remote Bluetooth device sending
malformed packets, causing denial of service on the host.
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