image/gif: EncodeAll panics when a palette is too large to encode

[topherbullock]

What version of Go are you using (go version)?

1.8.1

What operating system and processor architecture are you using (go env)?

darwin / amd64

What did you do?

Created a gif image with a palette ( either global or local color table for a frame ) containing > 256 colors.

https://play.golang.org/p/xfDZCcSIFF

What did you expect to see?

The EncodeAll function returns errors when image blocks are too large, on a zero-length gif, etc. As such, in this case since we know the gif can't be encoded due to the palette length being > 256, I would expect a similar error, along the lines of errors.New("gif: image palette is too large to encode") to match the style here

What did you see instead?

It panics here trying to read log2Lookup[-1] defined here

I have a change-set ready with the proposed functionality and accompanying tests.

[bradfitz]

Go 1.9 is fine too if trivial/safe.

/cc @nigeltao

[gopherbot]

CL https://golang.org/cl/42790 mentions this issue.

[topherbullock]

@bradfitz + @nigeltao Ah, okay.. Welp, that looks a lot like what I had ready to submit, so LGTM :D

[nigeltao]

Ah, I didn't read your final line that said you had a change-set ready. Sorry.

[topherbullock]

@nigeltao No worries 😄