Use the read and write buffers in DTLSv1_listen()

Rather than using init_buf we use the record layer read and write buffers
in DTLSv1_listen(). These seem more appropriate anyway and will help with
the next commit.

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/7375)

(cherry picked from commit 2fc4c77c)