[gnome-panel] status-notifier: add assert to silence coverity

[Alberts Muktupāvels <muktupavels src gnome org>]

commit f43ec89b6bef77a55c4a1835e9e0460b29f67711
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Sun Mar 8 14:29:43 2020 +0200

    status-notifier: add assert to silence coverity
    
    CID 1491641.

 modules/status-notifier/sn-item-v0.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/modules/status-notifier/sn-item-v0.c b/modules/status-notifier/sn-item-v0.c
index 63ab85ab5..a9e766fbf 100644
--- a/modules/status-notifier/sn-item-v0.c
+++ b/modules/status-notifier/sn-item-v0.c
@@ -209,6 +209,8 @@ get_surface (SnItemV0       *v0,
 
   g_list_free (pixmaps);
 
+  g_assert (surface != NULL || best != NULL);
+
   if (surface != NULL)
     return cairo_surface_reference (surface);