internal/cmd/cue-ast-print: do not panic on invalid values

For example, in SliceExpr, the High field can be a nil interface,
and that caused us to panic as reflect.Value.Elem returns a zero Value
in such a scenario. Thanks to Rudolf Farkas for the report and fix.

Note that we don't have any tests right now, as this tool
is used only for debugging purposes.

Closes #2771 as merged.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Id41797ea500b75bbc7a09f7f40cc9e51f5c0931a
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194721
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

internal/cmd/cue-ast-print/main.go

@@ -75,15 +75,17 @@ func (d *debugPrinter) value(v reflect.Value) {
 	if v.Kind() == reflect.Interface {
 		v = v.Elem()
 	}
-	// We print the original pointer type if there was one.
-	origType := v.Type()
-	v = reflect.Indirect(v)
 
+	// Indirecting a nil interface/pointer gives a zero value;
+	// stop as calling reflect.Value.Type on an invalid type would panic.
 	if !v.IsValid() {
-		// Indirecting a nil interface/pointer gives a zero value.
 		d.printf("nil")
 		return
 	}
+	// We print the original pointer type if there was one.
+	origType := v.Type()
+	v = reflect.Indirect(v)
+
 	t := v.Type()
 	switch t {
 	// Simple types which can stringify themselves.