std.crypto.tls: fix readIndirect returning 0 for application_data

[jedisct1]

The readIndirect function was incorrectly returning 0 when processing application_data, even though it had successfully read and decrypted data into the buffer.

Fixes #25428

Reported by @Southporter -- Thanks!

[squeek502]

This fix doesn't seem correct; readIndirect always returning 0 should be correct in terms of the stream and readVec APIs.

• stream returns how many bytes are written to w, and w is discarded so stream should always return 0
• readVec returns how many bytes were written to data, and data is discarded so readVec should always return 0

My preliminary guess is that the infinite loop is caused by the buffer being full during a stream call so the buffer just stays full forever and some Reader function loops infinitely. If so, doing a rebase in stream to make room might be a possible fix (here's an example of that sort of thing in the flate decompressor). Will look into it, though; that's just a guess at this point.

EDIT: It actually seems to be getting stuck during a readv syscall for me on Linux. The syscall is called and never returns? Not sure what would cause that.

EDIT#2: The hanging readv is coming from input.peek here so in theory this should be unrelated to tls.Client and more related to File.Reader (which is what input is in the reproduction)