Fix some NaNs in the context of the display of dbFS values when "View > Volume as dbFS" is checked. They occur during the display of the current value when a mixer fader or the volume knob of an instrument is pulled down completely.
The fix is to detect these cases and to display "-∞ dbFS".
Also fix a problem with the editor where dbFS values can be entered for volume knobs. When the knob is turned completely to the left and the amplification is 0 then the initially displayed value is set to -96 dBFS, i.e. the lower limit that is shown in the dialog. This is done because the dialog likely cannot handle displaying or entering "-∞".
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