Currently, the xml.Decoder's Token routine returns successfully for
XML input that does not properly close root start elements (and any
unclosed descendants). For example, all the following inputs

    <root>
    <root><foo>
    <root><foo></foo>

cause Token to return with nil and io.EOF, indicating a successful
parse.

This change fixes that. It leaves the semantics of RawToken intact.

Fixes #11405

Change-Id: I6f1328c410cf41e17de0a93cf357a69f12c2a9f7
Reviewed-on: https://go-review.googlesource.com/14315Reviewed-by: Nigel Tao <nigeltao@golang.org>