encoding: reset optional<> if it is uninitialized

encoding: reset optional<> if it is uninitialized

* should reset it, in case we reuse it after initializing it.
* initialize the value of `p` using the C++11 style initializer, so it
  is zero-initialized.
* revert 2a83ef3c which disables a warning of:
 ./include/encoding.h:317:7: warning: 't' may be used uninitialized in
    this function [-Wmaybe-uninitialized]
 where the `t` is the temporary variable for initializing the value of
 `p`.

Signed-off-by: Kefu Chai <kchai@redhat.com>