r.surf.idw: fix null value condition, https://trac.osgeo.org/grass/ticket/2671

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@65214 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.surf.idw/main.c

@@ -712,7 +712,7 @@ MELEMENT *row_lists(
 	Rast_get_c_row_nomask(fd, cell, row);
 
 	for (col = 0; col < cols; col++) {
-	    if (cell[col] != 0) {
+	    if (!Rast_is_c_null_value(&cell[col])) {
 		++(*npts);
 		Mptr = (MELEMENT *) G_malloc(sizeof(MELEMENT));
 		Mptr->x = col;