net: remove the alloc from WriteMsgUDPAddrPort

net: remove the alloc from WriteMsgUDPAddrPort

name                       old time/op    new time/op    delta
ReadWriteMsgUDPAddrPort-8    5.12µs ± 8%    4.59µs ± 3%  -10.19%  (p=0.000 n=10+9)

name                       old alloc/op   new alloc/op   delta
ReadWriteMsgUDPAddrPort-8     64.0B ± 0%     32.0B ± 0%  -50.00%  (p=0.000 n=10+10)

name                       old allocs/op  new allocs/op  delta
ReadWriteMsgUDPAddrPort-8      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)

Change-Id: Idf540b2f9f8035660305a0ab1cfc3e162569db63
Reviewed-on: https://go-review.googlesource.com/c/go/+/361257
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>