cmd/compile: don't expand invalid embedded interface

The invalid interface type will be reported already, so don't expand
that invalid one, which causes the compiler crashes.

Updates #43311

Change-Id: Ic335cfa74f0b9fcfd0929dc5fd31d9156a8f5f5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/298710
Trust: Cuong Manh Le <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>

Author: cuonglm

src/cmd/compile/internal/types/size.go

@@ -100,7 +100,7 @@ func expandiface(t *Type) {
 	}
 
 	for _, m := range t.Methods().Slice() {
-		if m.Sym != nil {
+		if m.Sym != nil || m.Type == nil {
 			continue
 		}
 

test/fixedbugs/issue20245.go

@@ -1,4 +1,4 @@
-// errorcheck
+// errorcheck -d=panic
 
 // Copyright 2017 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style

test/fixedbugs/issue22921.go

@@ -1,4 +1,4 @@
-// errorcheck
+// errorcheck -d=panic
 
 // Copyright 2020 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style

test/fixedbugs/issue27938.go

@@ -1,4 +1,4 @@
-// errorcheck
+// errorcheck -d=panic
 
 // Copyright 2018 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style