Prevent a potential crash when using manager hooks.

Found by me while poking at DPMA-127.
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Merged revisions 364841 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 364842 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@364844 65c4cc65-6c06-0410-ace0-fbb531ad65f3