Correctly handle dir connections the close during the pir lookup process

src/feature/hs/hs_pirprocess.c

@@ -431,5 +431,5 @@ hs_pirprocess_lookup_reqid(uint64_t reqid)
     memset(mapbuf, 0, DIGEST_LEN);
     memmove(mapbuf, &reqid, sizeof(reqid));
     entry = digestmap_get(pirprocess_reqid_map, mapbuf);
-    return entry->conn;
+    return entry ? entry->conn : NULL;
 }