Don't use any OR connection which sent us a CREATE_FAST cell for an EXTEND

Fix suggested by Nick Mathewson.

src/or/command.c

@@ -285,7 +285,13 @@ command_process_create_cell(cell_t *cell, or_connection_t *conn)
      * a CPU worker. */
     char keys[CPATH_KEY_MATERIAL_LEN];
     char reply[DIGEST_LEN*2];
+
     tor_assert(cell->command == CELL_CREATE_FAST);
+
+    /* Make sure we never try to use the OR connection on which we
+     * received this cell to satisfy an EXTEND request,  */
+    conn->is_connection_with_client = 1;
+
     if (fast_server_handshake(cell->payload, (uint8_t*)reply,
                               (uint8_t*)keys, sizeof(keys))<0) {
       log_warn(LD_OR,"Failed to generate key material. Closing.");