Ignore AttributeError when deleting temporary audio buffers

[mgeier]

Pure input/output streams only use one of data/out respectively, so we have to take care whether they even exist before we delete them.

Fixes#621.

[jared-v-hoyt]

Can we do the same thing with the following?

self.pop("data", None)
self.pop("out", None)

Not sure if there's a need to import contextlib here.

[mgeier]

No:

 self.pop("data", None)
^^^^^^^^
AttributeError: '_CallbackContext' object has no attribute 'pop'

You could do something like self.data = None, which I used originally. But then I replaced it in #605 to avoid errors of the Pyright static analyzer.

Not sure if there's a need to import contextlib here.

It's not strictly necessary, we could also just use try/except blocks, but they look more clumsy, and importing contextlib costs close to nothing.

It's also not strictly necessary to rely on catching exceptions at all (which is called "it's easier to ask for forgiveness than permission"), but instead check first whether the instance variables are defined or not (which is called "look before you leap").

There are many choices, and it's mostly a matter of taste.

[mgeier]

I'm merging this to avoid too many people tripping over this problem, but if you @jared-v-hoyt have (or anybody else has) any ideas for improvements, we can of course do them afterwards. PRs are welcome!