[Reviewer: Matt] Do more checks on the reg data from the HSS

Matt, can you please review this change to add more checks on the registration data returned by the HSS. 

Tested with UTs/LTs

Fixes #591
This code might be structured more cleanly as `if (!imss->first_node(...)) { ... return false; }  for (sp = imss->first_node(...); ...) { ... }\?