When calculating how much ciphertext from the stream can fit into
user and internal buffers we should also take into account ciphertext
data which are already in internal buffer.

Fixes: 15226

Tested with
[this](https://github.com/ziglang/zig/issues/15226#issuecomment-2218809140).
Using client with different read buffers until I, hopefully, understood
what is happening.

Not relevant to this fix, but this
[part](https://github.com/ziglang/zig/blob/95d9292a7a09ed883e65510ec054619747315c48/lib/std/crypto/tls/Client.zig#L988-L991)
is still mystery to me. Why we don't use free_size in buf_cap
calculation. Seems like rudiment from previous implementation without iovec.