Ignore retry packets that arrive too late

Ignore retry packets that arrive too late

RFC 9000 s 17.2.5.2 says

> After the client has received and processed an Initial or Retry packet
> from the server, it MUST discard any subsequent Retry packets that it
> receives.

We were checking for multiple Retry packets, but not if we had already
processed an Initial packet.

Fixes the assertion failure noted in
https://github.com/openssl/openssl/pull/22368#issuecomment-1765618884

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22411)