Don't clear the whole error stack when loading engines

Don't clear the whole error stack when loading engines

Loading the various built-in engines was unconditionally clearing the
whole error stack. During config file processing processing a .include
directive which fails results in errors being added to the stack - but
we carry on anyway. These errors were then later being removed by the
engine loading code, meaning that problems with the .include directive
never get shown.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/13311)