[desktop-bg] Fix memory leak during background change events

The string returned by eel_bg_get_desktop_color() needs to be freed by
the caller, which wasn't happening before.

Commit b9cf366 fixed the case where this function was being called
repeatedly, triggering this leak and causing it to rapidly fill system
memory.

libcaja-private/caja-directory-background.c

@@ -323,12 +323,15 @@ desktop_background_changed_cb (EelBackground *background,
 static gboolean
 desktop_background_prefs_change_event_idle_cb (EelBackground *background)
 {
+    gchar *desktop_color = NULL;
+
     eel_bg_load_from_gsettings (background,
                                 mate_background_preferences);
 
-    eel_background_set_color (background,
-                              eel_bg_get_desktop_color (background));
+    desktop_color = eel_bg_get_desktop_color (background);
+    eel_background_set_color (background, desktop_color);
 
+    g_free(desktop_color);
     g_object_unref (background);
 
     return FALSE;       /* remove from the list of event sources */