v.kernel: fix using mask

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@70941 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/v.kernel/main.c

@@ -567,7 +567,7 @@ int main(int argc, char **argv)
 
 	    for (col = 0; col < window.cols; col++) {
 		/* don't interpolate outside of the mask */
-		if (mask && mask[col] == 0) {
+		if (mask && Rast_is_c_null_value(&mask[col])) {
 		    Rast_set_d_null_value(&output_cell[col], 1);
 		    continue;
 		}