encoding/xml: avoid an allocation for tags without attributes

encoding/xml: avoid an allocation for tags without attributes

Before, an array of size 4 would always be allocated even if a tag
doesn't have any attributes. Now that array is allocated only if
needed.

benchmark              old allocs     new allocs     delta
BenchmarkUnmarshal     191            176            -8.5%

Change-Id: I4d214b228883d0a6e892c0d6eb00dfe2da84c116
Reviewed-on: https://go-review.googlesource.com/4160
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>