encoding/xml: remove unnecessary memory allocation in Unmarshal

benchmark              old ns/op     new ns/op     delta
BenchmarkUnmarshal     75256         72626         -3.49%

benchmark              old allocs     new allocs     delta
BenchmarkUnmarshal     259            219            -15.44%

Change-Id: I7fd30739b045e35b95e6ef6a8ef2f15b0dd6839c
Reviewed-on: https://go-review.googlesource.com/2758


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>