Fix an uninitialised read in conf_def.c

Fix an uninitialised read in conf_def.c

PR 8882 added a new field to the CONF structure. Unfortunately this
structure was created using OPENSSL_malloc() and the new field was not
explicitly initialised in the "init" function. Therefore when we came to
read it for the first time we got an uninitialised read.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/10428)