P11: Don't return int failure from a bool function
The functions return bool as per their prototype, but returning EINVAL
on failure meant that EINVAL (typically 22) was converted to 'true', so
a certificate that was not processable was considered valid.
Luckily this code only converts certificates into SSH public keys, so
there are no security implications.
Reviewed-by: Pavel Březina <email@example.com>
Reviewed-by: Sumit Bose <firstname.lastname@example.org>