bug(cloudflared): nil pointer deference on h2DictWriter Close() (#154)

Unlike other h2DictWriter methods, the Close() method does check whether
w.comp is nil.

This PR adds a check for non nil compressor before attempting to close

Bug: #141

h2mux/h2_dictionaries.go

@@ -542,8 +542,11 @@ func (w *h2DictWriter) Write(p []byte) (n int, err error) {
 }
 
 func (w *h2DictWriter) Close() error {
+	if w.comp != nil {
 		return w.comp.Close()
 	}
+	return nil
+}
 
 // From http2/hpack
 func http2ReadVarInt(n byte, p []byte) (remain []byte, v uint64, err error) {