Don't try to SSL_write() to a closing TLS connection

The function writing data to clients, tlsserverwr(), waits for the replyq->entries list to become non-empty by blocking on replyq->cond.

The function reading data from clietnts, tlsserverrd(), sets client->ssl = NULL when it wants to stop the writer.

tlsserverwr() should check client->ssl even in the case when the list is non-empty.