fix: add input validation to prevent remote crash from empty/malformed PDUs

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.

bumble/smp.py

@@ -215,6 +215,8 @@ class SMP_Command:
 
     @classmethod
     def from_bytes(cls, pdu: bytes) -> SMP_Command:
+        if not pdu:
+            raise ValueError("Empty SMP PDU")
         code = CommandCode(pdu[0])
 
         subclass = SMP_Command.smp_classes.get(code)