panic on embedding non-defined interface at `getEmbeddedInterfaceDefiningMethod`

[mcfog]

errcheck panics on following source file

package main

func main() {
	t := A(T{})
	t.X()
}

type T struct{}

func (t T) X() {}

type A interface {
	B // embeded
}

type B = interface { // B is not a defined type
	X()
}

embedded := interfaceT.Embedded(i) would be nil and panics on next line