Fix memory error for AudioFifo properties

[zzjjbb]

Validate self.ptr before access: uninitialized properties should return None rather than crash the Python interpreter

[zzjjbb]

I don't know why format, layout, and sample_rate are looked up in the same way, but AudioFifo().sample_rate will raise an AttributeError, while AudioFifo().format and AudioFifo().layout will crash the interpreter. Is there any magic caused by the docstrings in Cython?

[WyattBlue]

I think setting sample_rate to 0 and raising AttributeErrors is better than using None. Otherwise, this type widening would be a "breaking change"