Segfault prevention for setting default realms.

[Mats Erik Andersson]

It happens so that GNU Inetutils had code in the order

   shishi_realm_default_set(*handle, realm);

   shishi_init(handle);

Illegal order for sure, but it resulted in a segmentation
fault in libshishi. It should be an easy matter to perform
some sanity checks inside shishi_realm_default_set() in
order that the most obvious non-initiated handle be found,
thus returning an error status instead of crashing on a
segmentation fault.


Best regards,

  Mats Erik Andersson