gob: reduce the maximum message size

It was 2^31, but that could cause overflow and trouble.
Reduce it to 2^30 and add a TODO.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5562049

src/pkg/encoding/gob/decoder.go

@@ -75,7 +75,9 @@ func (dec *Decoder) recvMessage() bool {
 		dec.err = err
 		return false
 	}
-	if nbytes >= 1<<31 {
+	// Upper limit of 1GB, allowing room to grow a little without overflow.
+	// TODO: We might want more control over this limit.
+	if nbytes >= 1<<30 {
 		dec.err = errBadCount
 		return false
 	}

src/pkg/encoding/gob/gobencdec_test.go

@@ -547,7 +547,6 @@ func (a isZeroBugArray) GobEncode() (b []byte, e error) {
 }
 
 func (a *isZeroBugArray) GobDecode(data []byte) error {
-	println("DECODE")
 	if len(data) != len(a) {
 		return io.EOF
 	}