image/jpeg: invalid memory address or nil pointer dereference

[dvyukov]

Run the following program on the following input:

package main

import (
    "bytes"
    "image/jpeg"
    "io/ioutil"
    "os"
)

func main() {
    data, _ := ioutil.ReadFile(os.Args[1])
    img, err := jpeg.Decode(bytes.NewReader(data))
    if err != nil {
        return
    }
    var w bytes.Buffer
    err = jpeg.Encode(&w, img, nil)
    if err != nil {
        panic(err)
    }
}

https://drive.google.com/file/d/0B20Uwp8Hs1oCbTJPZW9zTXlORmM/view?usp=sharing

It crashes with:

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x18 pc=0x4566cc]

goroutine 1 [running]:
runtime.gopanic(0x4b2ac0, 0xc20800e260)
    src/runtime/panic.go:477 +0x3fe fp=0xc208041a70 sp=0xc2080419f0
runtime.panicmem()
    src/runtime/panic.go:42 +0x52 fp=0xc208041a98 sp=0xc208041a70
runtime.sigpanic()
    src/runtime/sigpanic_unix.go:24 +0x2bf fp=0xc208041ae8 sp=0xc208041a98
image/jpeg.(*decoder).processSOS(0xc208076000, 0x6, 0x0, 0x0)
    src/image/jpeg/scan.go:308 +0x11cc fp=0xc208041de8 sp=0xc208041ae8
image/jpeg.(*decoder).decode(0xc208076000, 0x7fd68274a1c0, 0xc208014450, 0x574300, 0x0, 0x0, 0x0, 0x0)
    src/image/jpeg/reader.go:619 +0xa4e fp=0xc208041e88 sp=0xc208041de8
image/jpeg.Decode(0x7fd68274a1c0, 0xc208014450, 0x0, 0x0, 0x0, 0x0)
    src/image/jpeg/reader.go:762 +0x69 fp=0xc208041ed0 sp=0xc208041e88
main.main()
    jpeg.go:12 +0x132 fp=0xc208041f90 sp=0xc208041ed0

My repository is on commit 8ac129e.