evdns: do not pass NULL to memcpy() in evdns_server_request_format_response()

In case of OPT pseudo-RR `class` field is treated as a requestor's UDP
payload size, and class will have 512 (DNS_MAX_UDP_SIZE), and data is
NULL:

  (gdb) p *item
  $4 = {
    next = 0x0,
    name = 0x602000000130 "",
    type = 41,
    class = 512,
    ttl = 0,
    is_name = 0 '\000',
    datalen = 0,
    data = 0x0
  }

And UBSAN will reports:

  ../evdns.c:2493:5: runtime error: null pointer passed as argument 2, which is declared to never be null
      #0 0x7ffff70b65bb in evdns_server_request_format_response ../evdns.c:2493
      #1 0x7ffff70b706b in evdns_server_request_respond ../evdns.c:2529
      #2 0x5555557975ab in regress_dns_server_cb ../test/regress_testutils.c:263
      #3 0x7ffff70a8489 in request_parse ../evdns.c:1576
      #4 0x7ffff70aa445 in server_udp_port_read ../evdns.c:1726
      #5 0x7ffff70ac5cc in server_port_ready_callback ../evdns.c:1849
      #6 0x7ffff6d3054c in event_persist_closure ../event.c:1645
      #7 0x7ffff6d311cd in event_process_active_single_queue ../event.c:1704
      #8 0x7ffff6d33258 in event_process_active ../event.c:1805
      #9 0x7ffff6d361b5 in event_base_loop ../event.c:2047
      #10 0x7ffff6d334ac in event_base_dispatch ../event.c:1839
      #11 0x555555739df2 in test_edns ../test/regress_dns.c:2639
      #12 0x5555557b9e96 in testcase_run_bare_ ../test/tinytest.c:173
      #13 0x5555557ba8f0 in testcase_run_one ../test/tinytest.c:333
      #14 0x5555557bc9a0 in tinytest_main ../test/tinytest.c:527
      #15 0x555555787faa in main ../test/regress_main.c:528
      #16 0x7ffff606c001 in __libc_start_main (/usr/lib/libc.so.6+0x27001)
      #17 0x55555569436d in _start (/src/le/libevent/.cmake-debug/bin/regress+0x14036d)

1 files changed, 6 insertions, 2 deletions

diff --git a/evdns.c b/evdns.c
index aeee5811..8df18568 100644
--- a/evdns.c
+++ b/evdns.c
@@ -2490,8 +2490,12 @@ evdns_server_request_format_response(struct server_request *req, int err)
 				APPEND16(item->datalen);
 				if (j+item->datalen > (off_t)buf_len)
 					goto overflow;
-				memcpy(buf+j, item->data, item->datalen);
-				j += item->datalen;
+				if (item->data) {
+					memcpy(buf+j, item->data, item->datalen);
+					j += item->datalen;
+				} else {
+					EVUTIL_ASSERT(item->datalen == 0);
+				}
 			}
 			item = item->next;
 		}