A fatal error used to happen when escassign-ing a multiple
function return to a single node. However, the situation
naturally appears when using "go f(g())" or "defer f(g())",
because g() is escassign-ed to sink.

Fixes #4529.

R=golang-dev, lvd, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/6920060