Only deref a reserved gateway session if we actually reserved one

Its perfectly acceptable to have a gateway session unreserved when we go to
first allocate one.  Unreffing the reserved gateway session - when its NULL -
will result in an assertion error.

This problem was caught by the Asterisk Test Suite (once we had enough of the
debugging flags enabled)
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Merged revisions 375797 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 375798 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@375799 65c4cc65-6c06-0410-ace0-fbb531ad65f3