chan_gtalk: Fix potential use of uninitialized variable.

Avoid potential use of idroster in gtalk_alloc() before it has been
initialized.
........

Merged revisions 359508 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 359509 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@359510 65c4cc65-6c06-0410-ace0-fbb531ad65f3