Merge pull request #16395 from FRRouting/mergify/bp/dev/10.1/pr-16365

isisd: fix crash when calculating the neighbor spanning tree based on the fragmented LSP (backport #16365)

isisd/isis_spf.c

@@ -873,6 +873,9 @@ static int isis_spf_process_lsp(struct isis_spftree *spftree,
 			    || (mt_router_info && !mt_router_info->overload));
 
 lspfragloop:
+	if (!lsp->tlvs)
+		return ISIS_OK;
+
 	if (lsp->hdr.seqno == 0) {
 		zlog_warn("%s: lsp with 0 seq_num - ignore", __func__);
 		return ISIS_WARNING;