usrloc: Avoid PN-related timer processing in "SQL/CacheDB Only" modes

Since the "PN trigger" logic described in RFC 8599 is currently strongly
tied to holding in-memory contacts, avoid any crashes or other
side-effects when running in "CacheDB Only" or "SQL Only" together with
PN support by simply avoiding linking contacts into the "PN trigger"
timer list.

Fixes #2902

(cherry picked from commit 60c1d54)

modules/usrloc/ul_timer.c

@@ -22,6 +22,7 @@
 #include "../../locking.h"
 #include "../../lib/list.h"
 
+#include "ul_mod.h"
 #include "ul_timer.h"
 #include "ul_evi.h"
 #include "ul_mi.h"
@@ -91,6 +92,9 @@ void start_refresh_timer(ucontact_t *ct)
 	struct list_head *el, *_;
 	ucontact_t *c;
 
+	if (!have_mem_storage())
+		return;
+
 	lock_get(ul_refresh_lock);
 	if (!list_empty(&ct->refresh_list))
 		list_del(&ct->refresh_list);