Fix AMI Redirect ExtraChannel not redirecting to the same exten and context.

The astman_get_header() never returns NULL so the check by the code for
NULL would never fail.

(closes issue ASTERISK-16974)
Reported by: Nuno Borges
Patches:
      0018325.patch (license #6116) patch uploaded by Nuno Borges (modified)
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Merged revisions 354835 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 354836 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@354837 65c4cc65-6c06-0410-ace0-fbb531ad65f3