Fix memory leak in 'manager show event' when command entered incorrectly

When the CLI command 'manager show event' was run incorrectly and its usage
instructions returned, a reference to the event container was leaked. This
would prevent the container from being reclaimed when Asterisk exits. We now
properly decrement the count on the ao2 object using the nifty RAII_VAR macro.

Thanks to Russell for helping me stumble on this, and Terry for writing that
ridiculously helpful macro.


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@377319 65c4cc65-6c06-0410-ace0-fbb531ad65f3