tls: fix segfault on destroy after partial read

OnRead() calls into JS land which can result in the SSL context object
being destroyed on return.  Check that `ssl_ != nullptr` afterwards.

Fixes: https://github.com/nodejs/node/issues/11885
PR-URL: https://github.com/nodejs/node/pull/11898


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>