[gimp] app: fix the same warning in gimp_levels_tool_config_notify()

[Michael Natterer <mitch src gnome org>]

commit 567ff4e136c7a1a577bec18ae053d41c5a883161
Author: Michael Natterer <mitch gimp org>
Date:   Sat Jan 20 16:01:20 2018 +0100

    app: fix the same warning in gimp_levels_tool_config_notify()

 app/tools/gimpcurvestool.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/app/tools/gimpcurvestool.c b/app/tools/gimpcurvestool.c
index 9b73672..71fd35c 100644
--- a/app/tools/gimpcurvestool.c
+++ b/app/tools/gimpcurvestool.c
@@ -611,7 +611,8 @@ gimp_curves_tool_config_notify (GimpFilterTool   *filter_tool,
   GIMP_FILTER_TOOL_CLASS (parent_class)->config_notify (filter_tool,
                                                         config, pspec);
 
-  if (! curves_tool->channel_menu)
+  if (! curves_tool->channel_menu ||
+      ! curves_tool->graph)
     return;
 
   if (! strcmp (pspec->name, "linear"))