The decoder incorrectly decoded the image to invalid RGBA values 错误地将图像解码为不合法的RGBA值

```golang
package main

import (
"fmt"
_ "github.com/chai2010/webp"
"image"
"os"
)

func main() {
file, err := os.Open("img.webp")
if err != nil {
fmt.Println("open file error:", err)
return
}
defer file.Close()

img, _, err := image.Decode(file)
if err != nil {
fmt.Println("decode error:", err)
return
}

switch v := img.(type) {
case *image.RGBA:
fmt.Println("This is RGBA")
case *image.NRGBA:
fmt.Println("This is NRGBA")
default:
fmt.Printf("Other type: %T\n", v)
}

bounds := img.Bounds()
width, height := bounds.Dx(), bounds.Dy()
for y := 0; y < height; y++ {
for x := 0; x < width; x++ {
r, g, b, a := img.At(x, y).RGBA()
r8, g8, b8 := uint8(r>>8), uint8(g>>8), uint8(b>>8)
a8 := uint8(a >> 8)

if a8 == 0 && (r8 != 0 || g8 != 0 || b8 != 0) {
fmt.Printf("Pixel(%d,%d): R=%d, G=%d, B=%d, A=%d\n", x, y, r8, g8, b8, a8)
}
}
}
}

```

```
Output：
This is RGBA
Pixel(176,0): R=0, G=1, B=0, A=0
Pixel(177,0): R=0, G=1, B=0, A=0
Pixel(178,0): R=0, G=1, B=0, A=0
Pixel(179,0): R=0, G=1, B=0, A=0
Pixel(180,0): R=0, G=1, B=0, A=0
Pixel(181,0): R=0, G=1, B=0, A=0
Pixel(182,0): R=0, G=1, B=0, A=0
Pixel(183,0): R=0, G=1, B=0, A=0
Pixel(184,0): R=0, G=1, B=0, A=0
Pixel(185,0): R=0, G=1, B=0, A=0
Pixel(186,0): R=0, G=1, B=0, A=0
Pixel(187,0): R=0, G=1, B=0, A=0
Pixel(188,0): R=0, G=1, B=0, A=0
Pixel(189,0): R=0, G=1, B=0, A=0
Pixel(190,0): R=0, G=1, B=0, A=0
Pixel(191,0): R=0, G=1, B=0, A=0
Pixel(192,0): R=108, G=109, B=104, A=0
Pixel(193,0): R=110, G=108, B=104, A=0
Pixel(194,0): R=111, G=107, B=104, A=0
Pixel(195,0): R=113, G=107, B=104, A=0
Pixel(196,0): R=116, G=105, B=104, A=0
Pixel(197,0): R=119, G=103, B=104, A=0
Pixel(198,0): R=121, G=103, B=104, A=0
Pixel(199,0): R=116, G=105, B=104, A=0
Pixel(200,0): R=120, G=126, B=117, A=0
Pixel(201,0): R=115, G=129, B=117, A=0
```

Reproducible image link: https://s2.loli.net/2025/08/22/V39dLFnrmzv27xu.webp

Start with the provided Go reproducer, the image.Decode entry point, and the linked WebP image to confirm the transparent pixels with nonzero RGB values. Trace the decoder path that produces the RGBA image and add a regression check for the reported pixels; done means decoding no longer yields the invalid RGBA values.

Written by the indexing model from the issue text.