3.0.16 regression: ALSA unusable as secondary input source

ALSA as secondary isource triggers the error Invalid url passed to the "input-slave" option and is ignored. This makes recording audio from an analog audio/video (V4L2) source impossible.

Regression is between 3.0.12 and 3.0.16, most probably caused by 6cbe6f5b in 3.0.13.

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