Prevent unnecessary recreation of resamplers in analog mode.

[tmiw]

Resolves #658 by fixing a bug where the AudioPipeline that does not have any steps inside it was unnecessarily recreating the output ResampleStep every time it processed a 20ms chunk of audio. This causes the state of the resampler to be lost, resulting in scratchy or dropped audio.

Note that the following must be true in order to observe this issue:

1. The sample rate for the digital input device ("From Radio to Computer") needs to be different than for the analog device ("From Computer to Speakers/Headphones"), and
2. FreeDV must be in analog mode. (Normal digital operation goes through a different flow inside the overall audio pipeline which doesn't seem to be affected by this issue.)