st-icon: check if gicon is null before unref.

Commit a6b49fe7d6
introduced a check for equality of priv->gicon vs gicon
and unref then return early if so. But if both are null,
we should not unref gicon. Only return.

https://bugzilla.gnome.org/show_bug.cgi?id=698863

src/st/st-icon.c

@@ -544,7 +544,8 @@ st_icon_set_icon_name (StIcon      *icon,
 
   if (g_icon_equal (priv->gicon, gicon)) /* do nothing */
     {
-      g_object_unref (gicon);
+      if (gicon)
+        g_object_unref (gicon);
       return;
     }