fix for sslecho in demos echoing garbage #18165

fix for sslecho in demos echoing garbage #18165

- getline does set &txbufp content at return, make sure it can be done.
  - fixes warning 'passing argument 1 of ‘getline’ from incompatible pointer type'
- remove OPENSSL_free on non allocated fixed size array
  - fixes 'free(): invalid pointer'

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18177)

(cherry picked from commit 3c0e8bc4a797d29b2152aebc6e687ddfa941160b)