Decoding CBOR tag 5 with `null` mantissa raises TypeError instead of CBORDecodeValueError

[coco1629]

Things to check first

• I have searched the existing issues and didn't find my bug already reported there

• I have checked that my bug is still present in the latest release

cbor2 version

5.8.0

Python version

3.9.18

What happened?

When decoding CBOR tag 5 (bigfloat) with a null mantissa, CBORDecoder.decode() raises TypeError instead of the expected CBORDecodeValueError. This is probably because the try/except in decode_bigfloat only covers self._decode() , leaving the Decimal(sig) * (2 ** Decimal(exp)) computation outside its scope.

How can we reproduce the bug?

data = bytes.fromhex("c58200f6")
obj = cbor2.CBORDecoder(io.BytesIO(data))
print(obj.decode())

trace:

Traceback (most recent call last):
  File "test.py", line 60, in <module>
    print(obj.decode())
TypeError: unsupported operand type(s) for *: 'NoneType' and 'decimal.Decimal'

Expected behavior: should raise CBORDecodeValueError, consistent with other invalid input.