txp_generate_stream_frames(): Set stream id in header early enough

txp_generate_stream_frames(): Set stream id in header early enough

Otherwise we will calculate an incorrect header
size for higher stream ids and won't fit the
frame into the packet.

Fixes #25417

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25928)