Fix bug in mod_deflate, which was passing calling deflate()

     without checkinig first whether it has something to deflate. (currently
     this causes deflate to generate a fatal error according to the zlib spec).
     PR 22259.
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git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@100966 13f79535-47bb-0310-9956-ffa450edef68