content_encoding is undefined if filename is "-"

Define var before use. Cutting edge computer science in
action.

S3/S3.py

@@ -413,6 +413,7 @@ def object_put(self, filename, uri, extra_headers = None, extra_label = ""):
 
         ## MIME-type handling
         content_type = self.config.mime_type
+        content_encoding = None
         if filename != "-" and not content_type and self.config.guess_mime_type:
             (content_type, content_encoding) = mime_magic(filename)
         if not content_type: