Skinny cleanup (mwi_event_cb).

Original was testing for d->session, setting and testing again (all nested).

Removed duplicate testing and restructured function to test/return and then
the main code.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@368681 65c4cc65-6c06-0410-ace0-fbb531ad65f3