rgw: stop crashing when an admin tries to assume a nonexistent role

rgw: stop crashing when an admin tries to assume a nonexistent role

An admin user sending an AssumeRole request for a role that doesn't
exist would crash rgw. The permission check failed but got
overridden because the caller was an admin, and the code that ran
next blindly assumed the role had already been loaded. Fixed by
loading the role directly instead of hoping it was done earlier.

Signed-off-by: Matthew N. Heler <matthew.heler@hotmail.com>
(cherry picked from commit a1f1811f50509f6986198b95d51802c328afa9af)