wire: Fix {read,write}Element *[4]byte case

A previous commit which intended to remove the no longer used *[4]byte special
case (for message header checksums) from writeElement inadvertently removed
this from readElement instead.  Remove it from the intended place, and restore
the readElement case to avoid hitting the slow path.

Author: jrick

wire/common.go

@@ -237,6 +237,14 @@ func readElement(r io.Reader, element interface{}) error {
 		*e = int64Time(time.Unix(int64(rv), 0))
 		return nil
 
+	// Message header checksum.
+	case *[4]byte:
+		_, err := io.ReadFull(r, e[:])
+		if err != nil {
+			return err
+		}
+		return nil
+
 	case *[6]byte:
 		_, err := io.ReadFull(r, e[:])
 		if err != nil {
@@ -503,14 +511,6 @@ func writeElement(w io.Writer, element interface{}) error {
 		}
 		return nil
 
-	// Message header checksum.
-	case *[4]byte:
-		_, err := w.Write(e[:])
-		if err != nil {
-			return err
-		}
-		return nil
-
 	// Block header final state.
 	case *[6]byte:
 		_, err := w.Write(e[:])