net: remove the alloc from ReadMsgUDPAddrPort

net: remove the alloc from ReadMsgUDPAddrPort

name                          old time/op    new time/op    delta
ReadWriteMsgUDPAddrPort-8       4.95µs ± 5%    4.55µs ± 1%    -7.96%  (p=0.016 n=5+4)

name                          old alloc/op   new alloc/op   delta
ReadWriteMsgUDPAddrPort-8        32.0B ± 0%      0.0B       -100.00%  (p=0.008 n=5+5)

name                          old allocs/op  new allocs/op  delta
ReadWriteMsgUDPAddrPort-8         1.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)

Change-Id: Ib968c6f2968926ec9a364dd52063cd0d7c29b10c
Reviewed-on: https://go-review.googlesource.com/c/go/+/360862
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>