[goffice] Fix a craher introduced with last commit.

[Jean Bréfort <jbrefort src gnome org>]

commit 016c35d53b03b231352272cec453ea2136e682a6
Author: Jean Brefort <jean brefort normalesup org>
Date:   Thu Jan 8 15:55:47 2015 +0100

    Fix a craher introduced with last commit.

 ChangeLog                     |    5 +++++
 NEWS                          |    1 +
 goffice/graph/gog-axis-line.c |    7 +++++--
 3 files changed, 11 insertions(+), 2 deletions(-)
---
diff --git a/ChangeLog b/ChangeLog
index 9ccab4c..416079d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@
 2015-01-08  Jean Brefort  <jean brefort normalesup org>
 
+       * goffice/graph/gog-axis-line.c (gog_axis_line_get_ticks): don't return
+       invalid ticks.
+
+2015-01-08  Jean Brefort  <jean brefort normalesup org>
+
        * goffice/graph/gog-axis-line.c: allow custom ticks and grid lines for axis
        lines. [#600482]
        * goffice/graph/gog-axis-line.h: ditto.
diff --git a/NEWS b/NEWS
index 012786c..ea9c4b5 100644
--- a/NEWS
+++ b/NEWS
@@ -6,6 +6,7 @@ Jean:
        * Implement canvas native scrolling. [#741394]
        * Don't use a data vector with no valid value. [#741910]
        * Allow custom ticks and grid lines for axis lines. [#600482]
+
 Morten:
        * Dead kitten salvage.
        * Fix canvas issues when removing items.
diff --git a/goffice/graph/gog-axis-line.c b/goffice/graph/gog-axis-line.c
index f7e3be5..d770dfa 100644
--- a/goffice/graph/gog-axis-line.c
+++ b/goffice/graph/gog-axis-line.c
@@ -2693,6 +2693,9 @@ GSF_CLASS_FULL (GogAxisLine, gog_axis_line,
 unsigned
 gog_axis_line_get_ticks (GogAxisLine *axis_line, GogAxisTick **ticks)
 {
-       *ticks = axis_line->ticks;
-       return axis_line->tick_nbr;
+       if (axis_line->custom_ticks[0].data != NULL && go_data_has_value (axis_line->custom_ticks[0].data)) {
+               *ticks = axis_line->ticks;
+               return axis_line->tick_nbr;
+       }
+       return 0;
 }