better error when calling s2n_offered_psk_list_choose_psk with empty PSK

[jmayclin]

Problem:

The following returns an internal error

struct s2n_offered_psk_list* list = ...

// this psk is allocated, but not actually set to any real values
struct s2n_offered_psk* uninitialized_psk = s2n_offered_psk_new();

// this returns an internal error.
s2n_offered_psk_list_choose_psk(empty_psk, 

We should not expect internal errors to be returned from library code unless something has gone very wrong.

Solution:

This should probably be some kind of usage error.