Catching AttributeError when repr(fifo) that hasn't had fifo.write(frame) called yet

[ekalosak]

This change makes debugging state errors easier by describing to the user why the state is incomplete.

Before the change:

>>> AudioFifo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "av/audio/fifo.pyx", line 15, in av.audio.fifo.AudioFifo.__repr__
  File "av/audio/fifo.pyx", line 183, in av.audio.fifo.AudioFifo.sample_rate.__get__
AttributeError: 'NoneType' object has no attribute 'sample_rate'

After the change:

>>> AudioFifo()
<av.AudioFifo uninitialized, use fifo.write(frame), at 0x108719650>