image/png: interface conversion: color.Color is color.NRGBA, not color.RGBA

[dvyukov]

Run the following program on the following input:

package main

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

func main() {
    data, _ := ioutil.ReadFile(os.Args[1])
    png.Decode(bytes.NewReader(data))
}

https://drive.google.com/file/d/0B20Uwp8Hs1oCMFhqU0I1eUNIaWM/view?usp=sharing

It crashes as:

panic: interface conversion: color.Color is color.NRGBA, not color.RGBA

goroutine 1 [running]:
runtime.gopanic(0x5176e0, 0xc208010300)
    /ssd/src/go10/src/runtime/panic.go:477 +0x3fe fp=0xc20803dc80 sp=0xc20803dc00
runtime.assertI2T(0x5260c0, 0x7f01b18fe378, 0xc20800f580, 0xc20803dcfc)
    /ssd/src/go10/src/runtime/iface.go:187 +0x1c0 fp=0xc20803dcb0 sp=0xc20803dc80
image/png.(*decoder).parsetRNS(0xc20800d500, 0xad, 0x0, 0x0)
    /ssd/src/go10/src/image/png/reader.go:268 +0x4d8 fp=0xc20803dd88 sp=0xc20803dcb0
image/png.(*decoder).parseChunk(0xc20800d500, 0x0, 0x0)
    /ssd/src/go10/src/image/png/reader.go:690 +0x9e7 fp=0xc20803de68 sp=0xc20803dd88
image/png.Decode(0x7f01b18fe2d8, 0xc208012420, 0x0, 0x0, 0x0, 0x0)
    /ssd/src/go10/src/image/png/reader.go:752 +0x247 fp=0xc20803df08 sp=0xc20803de68
main.main()
    /tmp/png.go:12 +0x10b fp=0xc20803df90 sp=0xc20803df08

I am on commit a5dec38