Resolve an invalid memory read on an event.

Valgrind pointed out that attempting to get an IE value from an event that has
no IEs produces an invalid memory read past the end of the event.  Thanks to
mmichelson for pointing the problem out to me and then testing the fix.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@269417 65c4cc65-6c06-0410-ace0-fbb531ad65f3