extmod/modssl_mbedtls: Clear sock member if error creating SSLSocket.

Otherwise if/when the finaliser runs for this newly created SSLSocket the
mbedtls state will be freed again.

Signed-off-by: Damien George <damien@micropython.org>

extmod/modssl_mbedtls.c

@@ -394,6 +394,7 @@ STATIC mp_obj_t ssl_socket_make_new(mp_obj_ssl_context_t *ssl_context, mp_obj_t
     return MP_OBJ_FROM_PTR(o);
 
 cleanup:
+    o->sock = MP_OBJ_NULL;
     mbedtls_ssl_free(&o->ssl);
     mbedtls_raise_error(ret);
 }