rhp4: use clear for zero reader (Chris)

rhp/v4/rpc.go

@@ -25,9 +25,7 @@ var zeros = zeroReader{}
 type zeroReader struct{}
 
 func (r zeroReader) Read(p []byte) (int, error) {
-	for i := range p {
-		p[i] = 0
-	}
+	clear(p)
 	return len(p), nil
 }