tls.Client: fix out of bounds panic

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](#15226 (comment)).
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.

Author: ianic

lib/std/crypto/tls/Client.zig

@@ -1012,7 +1012,7 @@ pub fn readvAdvanced(c: *Client, stream: anytype, iovecs: []const std.posix.iove
     // Cleartext capacity of output buffer, in records. Minimum one full record.
     const buf_cap = @max(cleartext_buf_len / max_ciphertext_len, 1);
     const wanted_read_len = buf_cap * (max_ciphertext_len + tls.record_header_len);
-    const ask_len = @max(wanted_read_len, cleartext_stack_buffer.len);
+    const ask_len = @max(wanted_read_len, cleartext_stack_buffer.len) - c.partial_ciphertext_end;
     const ask_iovecs = limitVecs(&ask_iovecs_buf, ask_len);
     const actual_read_len = try stream.readv(ask_iovecs);
     if (actual_read_len == 0) {