pki: Don't leak memory in pubkey to blob.

src/pki_crypto.c

@@ -855,12 +855,13 @@ ssh_string pki_publickey_to_blob(const ssh_key key)
 
             rc = buffer_add_ssh_string(buffer, e);
             if (rc < 0) {
-
-                ssh_string_burn(e);
-                ssh_string_free(e);
                 goto fail;
             }
 
+            ssh_string_burn(e);
+            ssh_string_free(e);
+            e = NULL;
+
             break;
 #endif
         case SSH_KEYTYPE_UNKNOWN: