dispatcher: fix several issues with empty destination sets

Upon (re)load, a dispatching set may be empty if none of its destinations
can be resolved. This patch fixes a couple of crashes which are directly
related to empty destination sets:

    * fix a bug in reindex_dests() where a single bogus destination
      would be allocated for each empty set
    * prevent ds_select_dst() from routing to empty sets

(cherry picked from commit 127b456)

modules/dispatcher/dispatch.c

@@ -291,6 +291,11 @@ int reindex_dests( ds_data_t *d_data)
 
 	for( sp=d_data->sets ; sp!= NULL ; sp->dlist=dp0, sp=sp->next )
 	{
+		if (sp->nr == 0) {
+			dp0 = NULL;
+			continue;
+		}
+
 		dp0 = (ds_dest_p)shm_malloc(sp->nr*sizeof(ds_dest_t));
 		if(dp0==NULL)
 		{
@@ -1292,6 +1297,11 @@ int ds_select_dst(struct sip_msg *msg, ds_select_ctl_p ds_select_ctl)
 		goto error;
 	}
 
+	if (idx->nr == 0) {
+		LM_DBG("destination set [%d] is empty!\n", idx->id);
+		goto error;
+	}
+
 	LM_DBG("set [%d]\n", ds_select_ctl->set);
 
 	ds_hash = 0;