struct hist: Also record the history for "NULL" objects

While the history of a "NULL" object isn't much of a rich/useful piece
of information, allowing it may help prevent errors in some cases, due
to "sh_push()" returning an unexpected NULL value => crash.

(cherry picked from commit cad9bd0)

lib/dbg/struct_hist.c

@@ -133,7 +133,7 @@ struct struct_hist *_sh_push(void *obj, struct struct_hist_list *list, int refs)
 {
 	struct struct_hist *sh, *last;
 
-	if (!obj || !list)
+	if (!list)
 		return NULL;
 
 	sh = shm_malloc(sizeof *sh);