encoding/json: Error() methods not handling null pointers gracefully

[havelock]

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

$ go version
# 1.19 - reproducible on https://go.dev/play/

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
# Go playground

What did you do?

https://go.dev/play/p/UzR0iDwh8eI

Failed type assertion generates a "special" zero value for json.SyntaxError here with its' msg being a nil pointer.
This causes a panic on trying to use its Error() method.

What did you expect to see?

Error() not causing null pointer dereference on an existing value that it's relatively simple to obtain; perhaps some simple "no error message" logics in https://cs.opensource.google/go/go/+/refs/tags/go1.19.4:src/encoding/json/scanner.go;l=52

This can be the case for other errors too, at least in this package.

https://go.dev/tour/methods/15

If not, ok will be false and t will be the zero value of type T, and no panic occurs.

It's also somewhat misleading that "zero value of type T" can mean two very different things; one generated by the process described in https://go.dev/ref/spec#The_zero_value which is the case here I believe, and one by json.SyntaxError{}. Apologies in advance if this goes beyond this bug report.

What did you see instead?

A segmentation fault due to null pointer dereference.

[seankhliao]

https://go.dev/doc/faq#nil_error

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