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)
........

Merged revisions 375797 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@375798 65c4cc65-6c06-0410-ace0-fbb531ad65f3