r.stream.distance elevations with value of -1 [Bug]

[csmithud]

Name of the addon
r.stream.distance

Describe the bug
r.stream.distance incorrectly converts elevation difference values that == -1 to be null values. -1 is used as a placeholder for null values in the source code but it is conceivable that -1 could be a valid elevation difference value.

To Reproduce

1. Create a stream raster and drainage direction raster based on an elevation dataset.
2. Create a proxy elevation raster where the stream raster values are == 1 and the non-stream areas == 0.
3. Run r.stream.distance with the following parameters: (direction = drainage direction raster, stream_rast = stream raster, elevation = proxy elevation raster, difference = output)

Expected behavior
One would expect that the output would be a raster with 0 along the stream line and -1 everywhere else but instead the function returns 0 along the stream line and nulls everywhere else.

System description (please complete the following information):

• Linux Ubuntu 22.04.1 jammy
• GRASS GIS version 7.8.1