r.distance: fix qsort cmp fn (see https://trac.osgeo.org/grass/ticket/3564)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@73139 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.distance/edges.c

@@ -150,7 +150,10 @@ static int cmp(const void *aa, const void *bb)
 {
     const struct CatEdgeList *a = aa, *b = bb;
 
-    return (int)(a->cat - b->cat);
+    if (a->cat < b->cat)
+	return -1;
+
+    return (a->cat > b->cat);
 }
 
 void sort_edge_list(struct Map *map)