Fix d2i_PublicKey() for EC keys

o2i_ECPublicKey() requires an EC_KEY structure filled with an EC_GROUP.

o2i_ECPublicKey() is called by d2i_PublicKey(). In order to fulfill the
o2i_ECPublicKey()'s requirement, d2i_PublicKey() needs to be called with
an EVP_PKEY with an EC_KEY containing an EC_GROUP.

However, the call to EVP_PKEY_set_type() frees any existing key structure
inside the EVP_PKEY, thus freeing the EC_KEY with the EC_GROUP that
o2i_ECPublicKey() needs.

This means you can't d2i_PublicKey() for an EC key...

The fix is to check to see if the type is already set appropriately, and
if so, not call EVP_PKEY_set_type().

Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8168)