encoding/xml: infinite loop when using `xml.NewTokenDecoder` with a custom `TokenReader` [Go 1.15]

[katiehockman]

The Decode, DecodeElement, and Skip methods of an xml.Decoder provided by xml.NewTokenDecoder may enter an infinite loop when operating on a custom xml.TokenReader which returns an EOF in the middle of an open XML element.

Thanks to Sam Whited for reporting this issue.
This issue is CVE-2021-27918.

[katiehockman]

Fixed by 91062c2

[katiehockman]

See #44913 for main issue