net: fix memory corruption in windows *netFD.ReadFrom
net: fix memory corruption in windows *netFD.ReadFrom

We must keep memory used by syscall.WSARecvFrom away from
garbage collector until after overlapped call is completed.

Fixes issue 2094.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/4817050

pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,gnanderson/go,tv42/old-go,tv42/old-go,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,gnanderson/go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,tv42/old-go,gnanderson/go,pombredanne/go-deleteme