fix: nil ptr when discarded key and value not present on range

gnovm/cmd/gno/testdata/test/issue_5664.txtar

@@ -0,0 +1,30 @@
+# range with several combinations of key and value (presence)
+
+gno run .
+
+-- main.gno --
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 2821
+package main
+
+type matrix struct {
+	e []int
+}
+
+func (a matrix) equal() bool {
+	for _ = range a.e {
+	}
+	for range a.e {
+	}
+	return true
+}
+
+func main() {
+	var a matrix
+	var i interface{}
+	i = true && a.equal()
+	_ = i
+}

gnovm/pkg/gnolang/type_check.go

@@ -829,8 +829,8 @@ func (x *RangeStmt) AssertCompatible(store Store, last BlockNode) {
 	if x.Op != ASSIGN {
 		return
 	}
-	if isBlankIdentifier(x.Key) && isBlankIdentifier(x.Value) {
-		// both "_"
+	if isBlankIdentifier(x.Key) && (x.Value == nil || isBlankIdentifier(x.Value)) {
+		// both "_" or key is "_" and value is not present
 		return
 	}
 	assertValidAssignLhs(store, last, x.Key)