BUG/MINOR: cfgcond: always set the error string on openssl_version checks

BUG/MINOR: cfgcond: always set the error string on openssl_version checks

Using openssl_version_before() with an invalid argument results in "(null)"
appearing in the error message due to -1 being returned without the error
message being filled. Let's always fill the error message on error.

This was introduced in 2.5 with commit 3aeb3f9347 ("MINOR: cfgcond:
implements openssl_version_atleast and openssl_version_before"), and
this fix must be backported to 2.6.