fmt: fix crash of %b on huge negative int64

The buffer had 64 bytes but needs one more for the sign.

Fixes #3510.

R=golang-dev, dave, dsymonds
CC=golang-dev
https://golang.org/cl/6011057

src/pkg/fmt/fmt_test.go

@@ -461,6 +461,9 @@ var fmttests = []struct {
 	// zero reflect.Value, which formats as <nil>.
 	// This test is just to check that it shows the two NaNs at all.
 	{"%v", map[float64]int{math.NaN(): 1, math.NaN(): 2}, "map[NaN:<nil> NaN:<nil>]"},
+
+	// Used to crash because nByte didn't allow for a sign.
+	{"%b", int64(-1 << 63), "-1000000000000000000000000000000000000000000000000000000000000000"},
 }
 
 func TestSprintf(t *testing.T) {

src/pkg/fmt/format.go

@@ -10,7 +10,7 @@ import (
 )
 
 const (
-	nByte = 64
+	nByte = 65 // %b of an int64, plus a sign.
 
 	ldigits = "0123456789abcdef"
 	udigits = "0123456789ABCDEF"