Don't attempt to link in an aggregate MWI subscription if an endpoint does not aggregate MWI.

Attempting to link a NULL object into an ao2 container had been benign previously, but since
enabling DO_CRASH in the testsuite, this is now causing a crash. It's better to be right
here anyway.
........

Merged revisions 410011 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@410012 65c4cc65-6c06-0410-ace0-fbb531ad65f3