extmod/modussl_mbedtls: Populate sock member right away in wrap_socket.

Otherwise the "sock" member may have an undefined value if wrap_socket
fails with an exception and exits early, and then if the finaliser runs it
will try to close an invalid stream object.

Fixes issue #3828.

extmod/modussl_mbedtls.c

@@ -124,6 +124,7 @@ STATIC mp_obj_ssl_socket_t *socket_new(mp_obj_t sock, struct ssl_args *args) {
     mp_obj_ssl_socket_t *o = m_new_obj(mp_obj_ssl_socket_t);
 #endif
     o->base.type = &ussl_socket_type;
+    o->sock = sock;
 
     int ret;
     mbedtls_ssl_init(&o->ssl);
@@ -171,7 +172,6 @@ STATIC mp_obj_ssl_socket_t *socket_new(mp_obj_t sock, struct ssl_args *args) {
         }
     }
 
-    o->sock = sock;
     mbedtls_ssl_set_bio(&o->ssl, &o->sock, _mbedtls_ssl_send, _mbedtls_ssl_recv, NULL);
 
     if (args->key.u_obj != MP_OBJ_NULL) {