Fix an assertion in the DTLS server code

Fix an assertion in the DTLS server code

This fixes an internal error alert from the server and
an unexpected connection failure in the release version,
but a failed assertion and a server crash in the
debug version.

Reproduce this issue with a DTLS server/client like that:

./openssl s_server -dtls -mtu 1500
./openssl s_client -dtls -maxfraglen 512

In the debug version a crash happens in the Server now:

./openssl s_server -dtls -mtu 1500
Using default temp DH parameters
ACCEPT
ssl/statem/statem_dtls.c:269: OpenSSL internal error: Assertion failed: len == written
Aborted (core dumped)

While in the release version the handshake exceeds the
negotiated max fragment size, and fails because of this:

$ ./openssl s_server -dtls -mtu 1500
Using default temp DH parameters
ACCEPT
ERROR
4057152ADA7F0000:error:0A0000C2:SSL routines:do_dtls1_write:exceeds max fragment size:ssl/record/rec_layer_d1.c:826:
shutting down SSL
CONNECTION CLOSED

From the client's point of view the connection fails
with an Internal Error Alert:

$ ./openssl s_client -dtls -maxfraglen 512
Connecting to ::1
CONNECTED(00000003)
40B76343377F0000:error:0A000438:SSL routines:dtls1_read_bytes:tlsv1 alert internal error:ssl/record/rec_layer_d1.c:613:SSL alert number 80

and now the connection attempt fails unexpectedly.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18093)