v.generalize: check that input map contains lines

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@62736 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/v.generalize/main.c

@@ -322,6 +322,11 @@ int main(int argc, char *argv[])
     if (Vect_open_old2(&In, map_in->answer, "", field_opt->answer) < 1)
 	G_fatal_error(_("Unable to open vector map <%s>"), map_in->answer);
 
+    if (Vect_get_num_primitives(&In, mask_type) == 0) {
+	G_warning(_("No lines found in input map <%s>"), map_in->answer);
+	Vect_close(&In);
+	exit(EXIT_SUCCESS);
+    }
     with_z = Vect_is_3d(&In);
 
     if (0 > Vect_open_new(&Out, map_out->answer, with_z)) {