cmd/compile: only check return for valid functions

cmd/compile: only check return for valid functions

CheckReturn uses fn.Type() unconditionally, so for invalid function,
fn.Type() will be nil, causes the compiler crashes.

Updates #43311

Change-Id: I4420dd296c72ea83986b38fbf2c7f51fa59757c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/298709
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>