stun: correct attribute string padding to match rfc

When sending the USERNAME attribute in an RTP STUN
response, the implementation in append_attr_string
passed the actual length, instead of padding it up
to a multiple of four bytes as required by the RFC
3489.  This change adds separate variables for the
string and padded attributed lengths, and performs
padding correctly.

Reported by: Thomas Arimont
Review: https://reviewboard.asterisk.org/r/4139/
........

Merged revisions 427874 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@427875 f38db490-d61c-443f-a65b-d21fe96a405b