g10: Make call to agent_scd_serialno more robust.
* g10/call-agent.c (agent_scd_serialno): Make sure that NULL is stored on error at r_serialno. * g10/card-util.c (card_status): Simplify freeing of seriaono. (factory_reset): Ditto.
This pattern is what we use with other functions returning an
allocated string and thus less surprising.