Fix return value of get_ie_str() and get_ie_str_hash() for non-existent IE.

I found this bug while developing a unit test for event allocation.  Testing
is awesome.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@245624 65c4cc65-6c06-0410-ace0-fbb531ad65f3