pki: Fix session_id hashing.

I don't understand why this code worked before. It was completly wrong.

src/pki.c

@@ -1088,11 +1088,11 @@ ssh_string ssh_pki_do_sign(ssh_session session,
         return NULL;
     }
 
-    session_id = ssh_string_new(SHA_DIGEST_LEN);
+    session_id = ssh_string_new(crypto->digest_len);
     if (session_id == NULL) {
         return NULL;
     }
-    ssh_string_fill(session_id, crypto->session_id, SHA_DIGEST_LEN);
+    ssh_string_fill(session_id, crypto->session_id, crypto->digest_len);
 
     ctx = sha1_init();
     if (ctx == NULL) {