Handling recordings with frequency 0

[kdarras]

All the Iceland recordings of the demonstration collection inexplicably include frequency 0 Hz. It causes errors when zooming into a selection that includes 0 Hz. Example: https://ecosound-web.de/ecosound_web/recording/show/21706068
Recordings open up fine in Audacity.
Uploading downsampled recordings partly fixes the issue: zooming is possible with the lower bound included i nthe selection, but only if the upper bound is not included.