cmd/link, reflect: regression in reflect.Method output

[dsnet]

Commit 44d3f89 caused a regression in the reflect package where-by some of the fields of the reflect.Method would be nil or invalid.

Consider the following program:

type Foo interface{}
type Bar struct{}

func (s *Bar) getFoo() Foo { return nil }

func main() {
    t := reflect.ValueOf(&Bar{}).Type()
    fmt.Println(t.Method(0))
}

On go1.6, this would print:
{getFoo main func(*main.Bar) main.Foo <func(*main.Bar) main.Foo Value> 0}

After 44d3f89, this would print:
{getFoo main <nil> <invalid Value> 0}

/cc @crawshaw @randall77 @ianlancetaylor

[randall77]

@mdempsky

[crawshaw]

This looks like a duplicate of #15673.