[balsa] main-window: Plug a GSlist leak

[Peter Bloomfield <peterb src gnome org>]

commit acd8d62a1370c3d76838aa991c6bcf6999382a96
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Tue May 5 17:52:55 2020 -0400

    main-window: Plug a GSlist leak

 src/main-window.c | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/src/main-window.c b/src/main-window.c
index b8a98fc2a..5cdd3147b 100644
--- a/src/main-window.c
+++ b/src/main-window.c
@@ -3201,6 +3201,11 @@ balsa_window_dispose(GObject * object)
         priv->open_mbnode_info_array = NULL;
     }
 
+    if (priv->activity_messages != NULL) {
+        g_slist_free_full(priv->activity_messages, g_free);
+        priv->activity_messages = NULL;
+    }
+
     balsa_app.in_destruction = TRUE;
     G_OBJECT_CLASS(balsa_window_parent_class)->dispose(object);