Avoid calling SSL_shutdown() in DTLS media transport if handshake isn't completed yet

[ming]

Calling SSL_shutdown() in DTLS media transport if handshake isn't completed will give the following error:

SSL  SSL_ERROR_SSL (Read): Level: 0 err: <336462231> <SSL routines-SSL_shutdown-shutdown while in init> len: 4000

This is similar to what's reported in r5648.

[ming]

In 6016:

Fixed #2206: Avoid calling SSL_shutdown() in DTLS media transport if handshake isn't completed yet

[ming]

In 6017:

Re #2206: Fixed incorrect variable