timer: only call the callback if not NULL

Fix a crash in osmo-sgsn.

Change-Id: I04d58d9580708cc0f6d0f4aa17d3e9f2c6235c8e
diff --git a/src/timer.c b/src/timer.c
index 10a0b95..cc6d5cc 100644
--- a/src/timer.c
+++ b/src/timer.c
@@ -241,7 +241,8 @@
 restart:
 	llist_for_each_entry(this, &timer_eviction_list, list) {
 		osmo_timer_del(this);
-		this->cb(this->data);
+		if (this->cb)
+			this->cb(this->data);
 		work = 1;
 		goto restart;
 	}