Fix #926: Fixed crash in GEOSInterpolate() when used with empty LineString.

[sir-sigurd]

https://trac.osgeo.org/geos/ticket/926

[dbaston]

@sir-sigurd semantics of empty geometries are always tricky. Why did you choose to have GEOSInterpolate return an empty point instead of NULL in this case? Is this what it does for other invalid cases such as non-linear inputs?

[sir-sigurd]

semantics of empty geometries are always tricky

I agree with it.

Is this what it does for other invalid cases such as non-linear inputs?

GEOSInterpolate returns NULL for non-linear inputs, but I don't think that empty LineString is invalid input for GEOSInterpolate. I think it should work similar to GEOSGetCentroid which returns empty point for empty input geometries, also if I'm not missing something GEOSGetCentroid(ls) == GEOSInterpolateNormalized(ls, 0.5) for any ls with two points and I think it should also be true for empty ls.

[dbaston]

Sounds logical to me. Any objections, @strk or others?

[strk]

I agree on the empty semantic