Don't set *pval to NULL in ASN1_item_ex_new.

While *pval is usually a pointer in rare circumstances it can be a long
value. One some platforms (e.g. WIN64) where
sizeof(long) < sizeof(ASN1_VALUE *) this will write past the field.

*pval is initialised correctly in the rest of ASN1_item_ex_new so setting it
to NULL is unecessary anyway.

Thanks to Julien Kauffmann for reporting this issue.

Reviewed-by: Richard Levitte <levitte@openssl.org>