Initialize file descriptors for dummy channels to -1.

Dummy channels usually aren't read from, but functions like SHELL and CURL
use autoservice on the channel.

(closes issue ASTERISK-20283)
Reported by: Gareth Palmer
Patches:
      svn-371580.patch (license #5169) patch uploaded by Gareth Palmer (modified)
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Merged revisions 371888 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 371890 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 371891 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@371892 65c4cc65-6c06-0410-ace0-fbb531ad65f3