[gimp] app: don't crash in gimp_tool_info_get_description()

[Michael Natterer <mitch src gnome org>]

commit 3f6e9e429dfb0ea4b557027a9b1e023e3cbf476f
Author: Michael Natterer <mitch gimp org>
Date:   Wed Jul 5 18:34:37 2017 +0200

    app: don't crash in gimp_tool_info_get_description()
    
    Found and tracked down by Elle Stone.

 app/core/gimptoolinfo.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/app/core/gimptoolinfo.c b/app/core/gimptoolinfo.c
index 23e041b..5edb83a 100644
--- a/app/core/gimptoolinfo.c
+++ b/app/core/gimptoolinfo.c
@@ -196,7 +196,8 @@ gimp_tool_info_get_description (GimpViewable  *viewable,
 {
   GimpToolInfo *tool_info = GIMP_TOOL_INFO (viewable);
 
-  *tooltip = g_strdup (tool_info->tooltip);
+  if (tooltip)
+    *tooltip = g_strdup (tool_info->tooltip);
 
   return g_strdup (tool_info->label);
 }