http: Fix spurious ERROR message in responses with no content.

Backport -r411687 and fix the fix because content_length is the length of
out plus the length of the file controlled by fd.

When a response has an out content length of 0, fwrite would be called to
write a buffer with no data in it.  This resulted in the following classic
error message:

  [Apr  3 11:49:17] ERROR[26421] http.c: fwrite() failed: Success

This patch makes it so that we only attempt to write the content of out if
the out string is non-zero.
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Merged revisions 412922 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 412923 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@412924 f38db490-d61c-443f-a65b-d21fe96a405b