encoding/base64: Decode reports incorrect error index

[josselin-c]

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

1.8

What did you do?

Fuzzed the mainline implementation against a WIP.
Found some cases where the error reported by mainline has not the correct index: https://play.golang.org/p/koSIFlLi4W

What did you expect to see?

"illegal base64 data at input byte 1"
"illegal base64 data at input byte 1"

What did you see instead?

"illegal base64 data at input byte 3"
"illegal base64 data at input byte 2"

I'll send a CL

[gopherbot]

CL https://golang.org/cl/37837 mentions this issue.