ASN1_primitive_new NULL param handling

ASN1_primitive_new takes an ASN1_ITEM * param |it|. There are a couple
of conditional code paths that check whether |it| is NULL or not - but
later |it| is deref'd unconditionally. If |it| was ever really NULL then
this would seg fault. In practice ASN1_primitive_new is marked as an
internal function in the public header file. The only places it is ever
used internally always pass a non NULL parameter for |it|. Therefore, change
the code to sanity check that |it| is not NULL, and remove the conditional
checking.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>