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.