encoding/json: use SetBytes in UnmarshalReuse benchmark

encoding/json: use SetBytes in UnmarshalReuse benchmark

This was the only benchmark missing the SetBytes call, as spotted
earlier by Bryan.

It's not required to make the benchmark useful, but it can still be a
good way to see how its speed is affected by the reduced allocations:

name                  time/op
CodeUnmarshal-8        12.1ms ± 1%
CodeUnmarshalReuse-8   11.4ms ± 1%

name                  speed
CodeUnmarshal-8       161MB/s ± 1%
CodeUnmarshalReuse-8  171MB/s ± 1%

name                  alloc/op
CodeUnmarshal-8        3.28MB ± 0%
CodeUnmarshalReuse-8   1.94MB ± 0%

name                  allocs/op
CodeUnmarshal-8         92.7k ± 0%
CodeUnmarshalReuse-8    77.6k ± 0%

While at it, remove some unnecessary empty lines.

Change-Id: Ib2bd92d5b3237b8f3092e8c6f863dab548fee2f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/170938
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>