built-in TCP is currently not working with WolfSSL (builds fine)

[scaprile]

https://github.com/cesanta/mongoose/actions/runs/18417997233/job/52486102662

This branch originated as a fix to the buffer bootstrap issue that works fine for buffered libs, but not for WolfSSL; it refuses to read encrypted data if its destination pointer is NULL (and length is 0).
Some tests pass, but this particular sequence fails, suggesting the prior failure check might need some cleanup (that is not needed for OpenSSL)

Note: disregard enthusiast submitted "fixes" below, those were even unrelated back then