[gnumeric] xlsx: avoid criticals on exporting unnamed trend line.

[Morten Welinder <mortenw src gnome org>]

commit cf501a76f38645bdfaf5fbb5371d728b19f33e0a
Author: Morten Welinder <terra gnome org>
Date:   Thu Oct 26 19:07:34 2017 -0400

    xlsx: avoid criticals on exporting unnamed trend line.

 NEWS                               |    1 +
 plugins/excel/ChangeLog            |    3 ++-
 plugins/excel/xlsx-write-drawing.c |    8 +++++---
 3 files changed, 8 insertions(+), 4 deletions(-)
---
diff --git a/NEWS b/NEWS
index 7da61ed..0a206b0 100644
--- a/NEWS
+++ b/NEWS
@@ -15,6 +15,7 @@ Morten:
        * xlsx import improvements for contour graphs.
        * xlsx export improvements for contour graphs.
        * Fix xlsx export of trend line equation.
+       * Avoid critical on exporting unnamed xlsx trend line.
 
 --------------------------------------------------------------------------
 Gnumeric 1.12.35
diff --git a/plugins/excel/ChangeLog b/plugins/excel/ChangeLog
index 775b553..ca52c70 100644
--- a/plugins/excel/ChangeLog
+++ b/plugins/excel/ChangeLog
@@ -3,7 +3,8 @@
        * xlsx-write-drawing.c (xlsx_write_axis): Make axis tag for
        contour charts match what XL wants.  Even if it isn't obvious why.
        (xlsx_write_one_chart): Save a view3D for contour charts.
-       (xlsx_write_one_plot): Fix writing trendline equation.
+       (xlsx_write_one_plot): Fix writing trendline equation.  Don't
+       write a trendline name if there isn't one.
 
 2017-10-24  Morten Welinder  <terra gnome org>
 
diff --git a/plugins/excel/xlsx-write-drawing.c b/plugins/excel/xlsx-write-drawing.c
index 5de801b..7f901b1 100644
--- a/plugins/excel/xlsx-write-drawing.c
+++ b/plugins/excel/xlsx-write-drawing.c
@@ -1113,9 +1113,11 @@ xlsx_write_one_plot (XLSXWriteState *state, GsfXMLOut *xml,
 
                        gsf_xml_out_start_element (xml, "c:trendline");
                        dat = gog_dataset_get_dim (GOG_DATASET (trend), -1);
-                       name = go_data_get_scalar_string (dat);
-                       gsf_xml_out_simple_element (xml, "c:name", name);
-                       g_free (name);
+                       if (dat) {
+                               char *name = go_data_get_scalar_string (dat);
+                               gsf_xml_out_simple_element (xml, "c:name", name);
+                               g_free (name);
+                       }
                        xlsx_write_go_style (xml, state, go_styled_object_get_style (GO_STYLED_OBJECT 
(trend)));
                        xlsx_write_chart_cstr_unchecked (xml, "c:trendlineType", trend_type);