go/types: more robust operand printing

Not a fix but useful for further debugging, and safe.

For #18643.

Change-Id: I5fb4f4a8662007a26e945fff3986347855f00eab
Reviewed-on: https://go-review.googlesource.com/46393
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/go/types/operand.go

@@ -122,7 +122,8 @@ func operandString(x *operand, qf Qualifier) string {
 	case invalid, novalue, builtin, typexpr:
 		// no type
 	default:
-		// has type
+		// should have a type, but be cautious (don't crash during printing)
+		if x.typ != nil {
 			if isUntyped(x.typ) {
 				buf.WriteString(x.typ.(*Basic).name)
 				buf.WriteByte(' ')
@@ -130,6 +131,7 @@ func operandString(x *operand, qf Qualifier) string {
 			}
 			hasType = true
 		}
+	}
 
 	// <mode>
 	buf.WriteString(operandModeString[x.mode])