s_server: correctly handle 2^14 byte long records

s_server: correctly handle 2^14 byte long records

as the code uses BIO_gets, and it always null terminates the
strings it reads, when it reads a record 2^14 byte long, it actually
returns 2^14-1 bytes to the calling application, in general it returns
size-1 bytes to the caller

This makes the code sub-optimal (as every 2^14 record will need two
BIO_gets() calls) and makes it impossible to use -rev option to test
all plaintext lengths (like in openssl#15706)

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17538)