encoding/gob: fix Debug to properly print uint

Fix debugger printing of uint that mistakenly
invoked .int64() instead of .uint64()

Fixes #21392

Change-Id: I107a7e87e0efbb06303c1e627dee76c369f75d1e
Reviewed-on: https://go-review.googlesource.com/54750Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/gob/codec_test.go

@@ -1321,6 +1321,7 @@ func TestUnexportedFields(t *testing.T) {
 var singletons = []interface{}{
 	true,
 	7,
+	uint(10),
 	3.2,
 	"hello",
 	[3]int{11, 22, 33},

src/encoding/gob/debug.go

@@ -594,7 +594,7 @@ func (deb *debugger) printBuiltin(indent tab, id typeId) {
 		x := deb.int64()
 		fmt.Fprintf(os.Stderr, "%s%d\n", indent, x)
 	case tUint:
-		x := deb.int64()
+		x := deb.uint64()
 		fmt.Fprintf(os.Stderr, "%s%d\n", indent, x)
 	case tFloat:
 		x := deb.uint64()