Avoid new'ing memory that will be overwritten by assignment.

name              old time/op    new time/op    delta
Call-4               160ns ± 4%     155ns ± 2%  -3.19%        (p=0.003 n=10+10)
FieldByName1-4      94.5ns ± 2%    95.2ns ± 1%  +0.65%          (p=0.026 n=9+9)
FieldByName2-4      3.09µs ± 4%    3.13µs ± 2%    ~           (p=0.165 n=10+10)
FieldByName3-4      19.8µs ± 1%    19.9µs ± 1%    ~            (p=0.395 n=10+8)
InterfaceBig-4      11.6ns ± 0%    11.7ns ± 0%  +0.86%          (p=0.000 n=8+9)
InterfaceSmall-4    11.7ns ± 0%    11.7ns ± 0%    ~     (all samples are equal)
New-4               26.6ns ± 0%    26.4ns ± 0%  -0.64%         (p=0.000 n=10+9)

name              old alloc/op   new alloc/op   delta
Call-4              0.00B ±NaN%    0.00B ±NaN%    ~     (all samples are equal)

name              old allocs/op  new allocs/op  delta
Call-4               0.00 ±NaN%     0.00 ±NaN%    ~     (all samples are equal)

Change-Id: I12c85d4e65245598669dd6f66beb0744ec9b9d6d
Reviewed-on: https://go-review.googlesource.com/28011
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>