encoding/xml: fix race using finfo.parents in s.trim

This race was identified in #9796, but a sequence of fixes
proposed in golang.org/cl/4152 were rolled into
golang.org/cl/5910 which both fixed the race and
modified the name space behavior.

We rolled back the name space changes and lost the race fix.

Fix the race separate from the name space changes,
following the suggestion made by Roger Peppe in
https://go-review.googlesource.com/#/c/4152/7/src/encoding/xml/marshal.go@897

Fixes #9796.
Fixes #11885.

Change-Id: Ib2b68982da83dee9e04db8b8465a8295259bba46
Reviewed-on: https://go-review.googlesource.com/12687
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

src/encoding/xml/marshal.go

@@ -945,7 +945,7 @@ func (s *parentStack) trim(parents []string) error {
 			return err
 		}
 	}
-	s.stack = parents[:split]
+	s.stack = s.stack[:split]
 	return nil
 }