tls: fix assertion when ssl is destroyed at read

`maybeInitFinished()` can emit the 'secure' event which
in turn destroys the connection in case of authentication
failure and sets `this.pair.ssl` to `null`.

If such condition appeared after non-empty read - loop will continue
and `clearOut` will be called on `null` object instead of
`crypto::Connection` instance. In the result, following assertion was
thrown:

    ERROR: Error: Hostname/IP doesn't match certificate's altnames
    Assertion failed: handle->InternalFieldCount() > 0

fix #5756