adaptive: avoid a division by 0

rate.time may not be set in some cases.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/adaptive/http/Chunk.cpp

@@ -354,7 +354,7 @@ void HTTPChunkBufferedSource::bufferize(size_t readsize)
         }
     }
 
-    if(rate.size)
+    if(rate.size && rate.time)
     {
         connManager->updateDownloadRate(sourceid, rate.size, rate.time);
     }