Added additional checks for mailbox / password beginning with '*' character

Added additional checks for mailbox / password beginning with '*' character

A bug existed such that if a user entered a password with '*', and the extension 'a' did not exist, an invalid mailbox would be created and the user authenticated.  The code was changed to prevent this from occurring, and to prevent users from having mailboxes or passwords defined that begin with the '*' character.

(closes issue ASTERISK-17443)
Reported by: Kevin Scott Adams
Tested by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/1316/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@327852 65c4cc65-6c06-0410-ace0-fbb531ad65f3