r.kappa: fix qsort callback (partial fix for https://trac.osgeo.org/grass/ticket/3564)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@72729 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.kappa/prt_mat.c

@@ -206,5 +206,8 @@ static int longcomp(const void *aa, const void *bb)
     const long *a = aa;
     const long *b = bb;
 
-    return (*a - *b);
+    if (*a != *b)
+	return (*a > *b ? 1 : -1);
+
+    return 0;
 }