[goffice] AxisColorMap: add precondition.

[Morten Welinder <mortenw src gnome org>]

commit 40e5905a62d98285d85cc8343e2e240cecd2e603
Author: Morten Welinder <terra gnome org>
Date:   Tue May 24 17:50:13 2016 -0400

    AxisColorMap: add precondition.

 goffice/graph/gog-axis-color-map.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/goffice/graph/gog-axis-color-map.c b/goffice/graph/gog-axis-color-map.c
index 3003b44..1f93f66 100644
--- a/goffice/graph/gog-axis-color-map.c
+++ b/goffice/graph/gog-axis-color-map.c
@@ -559,7 +559,10 @@ gog_axis_color_map_to_cairo (GogAxisColorMap const *map, cairo_t *cr,
                              unsigned discrete, gboolean horizontal,
                              double width, double height)
 {
-       unsigned i, max = gog_axis_color_map_get_max (map);
+       unsigned i, max;
+
+       g_return_if_fail (GOG_IS_AXIS_COLOR_MAP (map));
+       max = gog_axis_color_map_get_max (map);
        if (discrete) {
                GOColor color;
                double t0, maxt, step, start, scale = 1;