Don't attempt to purge sessions when no sessions exist

Manager's tcp/tls objects have a periodic function that purge old manager
sessions periodically.  During shutdown, the underlying container holding
those sessions can be disposed of and set to NULL before the tcp/tls periodic
function is stopped.  If the periodic function fires, it will attempt to
iterate over a NULL container.

This patch checks for whether or not the sessions container exists before
attempting to purge sessions out of it.  If the sessions container is NULL,
we simply return.

Note that this error was also caught by the Asterisk Test Suite.
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Merged revisions 375800 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 375801 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 375802 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@375803 65c4cc65-6c06-0410-ace0-fbb531ad65f3