Fix nil listener callback

[222twotwotwo]

What this PR does:

• fix the nil listener guard in handlePackage error cleanup by requiring both session and listener before calling OnError
• guard OnClose when the listener is nil so the same cleanup path does not panic one line later
• add a regression test for the nil-listener error cleanup path

Which issue(s) this PR fixes:

Fixes #132

Special notes for your reviewer:

The cleanup path used s != nil || s.listener != nil, so a non-nil session with a nil listener still attempted to call s.listener.OnError(...) and could panic while handling an existing read/decode error.

Does this PR introduce a user-facing change?:

NONE

[AlexStocks]

这个问题，给上游 alexstocks/getty 也来个 PR @222twotwotwo

[222twotwotwo]

这个问题，给上游 alexstocks/getty 也来个 PR @222twotwotwo

done.