Fix another, more subtle, case of bug 17150.

We need to make sure that the corresponding sd and ei match in their
certificates.

changes/bug17150

@@ -1,5 +1,6 @@
   o Minor bugfixes (directory warnings):
     - When fetching extrainfo documents, compare their SHA256 digests
+      and Ed25519 signing key certificates
       with the routerinfo that led us to fetch them, rather than
       with the most recent routerinfo. Otherwise we generate many
       spurious warnings about mismatches. Fixes bug 17150; bugfix

src/or/routerlist.c

@@ -4949,7 +4949,7 @@ routerinfo_incompatible_with_extrainfo(const routerinfo_t *ri,
     goto err; /* different servers */
   }
 
-  if (! tor_cert_opt_eq(ri->signing_key_cert, ei->signing_key_cert)) {
+  if (! tor_cert_opt_eq(sd->signing_key_cert, ei->signing_key_cert)) {
     if (msg) *msg = "Extrainfo signing key cert didn't match routerinfo";
     goto err; /* different servers */
   }