go/printer: don't crash if ast.FuncType.Params is nil

go/printer: don't crash if ast.FuncType.Params is nil

The go/ast comment for FuncType.Params says that the field may be nil.
Make sure the printer accepts such a value. The go/parser always sets
the field (to provide parenthesis position information), but a program
creating a Go AST from scatch may not.

Added corresponding test case.

Fixes #3870.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6448060