Correctly track the original length when generating a stream frame

Correctly track the original length when generating a stream frame

txp_generate_stream_frames() plans chunks of data to send via the
function txp_plan_stream_chunk(). That function may clamp the amount in
the chunk due to flow control, even though there is more available to send.

We should take this into account when deciding whether or not to try
serializing the next chunk.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22601)