aubuf: preallocate mbuf frames to avoid malloc

[sreimers]

malloc and free calls should be avoided in realtime audio context as much as possible. this makes aubuf write/read calls ~1.8x times faster.

[cspiel1]

I'd like to vote again for this improvement (maybe simplified somehow). After planned v2.11.0?

[sreimers]

Yes, will take a look into this for next release.