Ensure that a null vmexten does not cause a segfault

When sip_send_mwi_to_peer was modified recently to avoid deadlocks, vmexten
was not expected to be null.  This change handles that situation to avoid
a segfault.
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Merged revisions 345063 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 345064 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@345065 65c4cc65-6c06-0410-ace0-fbb531ad65f3