Don't trust SNI names, fixes #1274

If an item doesn't have a well-formed name, it will not be added to the
tray.

include/swaybar/tray/sni.h

@@ -28,6 +28,7 @@ void sni_icon_ref_free(struct sni_icon_ref *sni_ref);
 
 /**
  * Will return a new item and get its icon. (see warning below)
+ * May return `NULL` if `name` is not valid.
  */
 struct StatusNotifierItem *sni_create(const char *name);