Fix NULL pointer deref when parsing the stable section

Fix NULL pointer deref when parsing the stable section

When parsing the stable section of a config such as this:
openssl_conf = openssl_init
[openssl_init]
stbl_section = mstbl
[mstbl]
id-tc26 = min

Can lead to a SIGSEGV, as the parsing code doesnt recognize min as a
proper section name without a trailing colon to associate it with a
value.  As a result the stack of configuration values has an entry with
a null value in it, which leads to the SIGSEGV in do_tcreate when we
attempt to pass NULL to strtoul.

Fix it by skipping any entry in the config name/value list that has a
null value, prior to passing it to stroul

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22988)