when read public key fails trigger a crash

[vletoux]

(when sc_card_operations.read_public_key is implemented ?) it returns an error when the public key is read, the assertion in sc_pkcs15_dup_pubkey for key == NULL triggers a crash.
This can be reproduced with pkcs11-tool -O

Cause:
in https://github.com/OpenSC/OpenSC/blob/master/src/pkcs11/framework-pkcs15.c#L775 pubkey->pub_data is NULL (called from _pkcs15_create_typed_objects)