Make EVP_PKEY_asn1_new() stricter with its input

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6880)

(cherry picked from commit 38eca7fe)