[gnumeric] xlsx: don't try to write missing axis.

[Morten Welinder <mortenw src gnome org>]

commit 2df935cc0672c7a3cbea3a73877cc9c302c745ac
Author: Morten Welinder <terra gnome org>
Date:   Mon Apr 27 17:41:00 2015 -0400

    xlsx: don't try to write missing axis.

 plugins/excel/xlsx-write-drawing.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/plugins/excel/xlsx-write-drawing.c b/plugins/excel/xlsx-write-drawing.c
index 99a9be5..a635a87 100644
--- a/plugins/excel/xlsx-write-drawing.c
+++ b/plugins/excel/xlsx-write-drawing.c
@@ -1162,8 +1162,10 @@ xlsx_write_one_plot (XLSXWriteState *state, GsfXMLOut *xml,
        for (i = 0; i < 3; i++) {
                if (axis_type[i] != GOG_AXIS_UNKNOWN) {
                        GogAxis *axis = gog_plot_get_axis (GOG_PLOT (plot), axis_type[i]);
-                       xlsx_write_chart_uint (xml, "c:axId", xlsx_get_axid (state, axis));
-                       axes = g_slist_append (axes, axis);
+                       if (axis) {
+                               xlsx_write_chart_uint (xml, "c:axId", xlsx_get_axid (state, axis));
+                               axes = g_slist_append (axes, axis);
+                       }
                }
        }