Only call dtls1_start_timer() once

Only call dtls1_start_timer() once

The function dtls1_handle_timeout() calls dtls1_double_timeout() which
was calling dtls1_start_timer(). However dtls1_start_timer() is also
called directly by dtls1_handle_timeout(). We only need to start the timer
once.

Fixes #15561

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15595)

(cherry picked from commit f570d33b02d824e2a3f676f718c4692572f45333)