stack.c: add missing direct error reporting and improve coding style

stack.c: add missing direct error reporting and improve coding style

Doing so, had to fix sloppiness in using the stack API in crypto/conf/conf_def.c,
ssl/ssl_ciph.c, ssl/statem/statem_srvr.c, and mostly in test/helpers/ssltestlib.c.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from https://github.com/openssl/openssl/pull/18918)