compiler: allow to use _ in constants

When `_` is unused it can be omitted from constant values mapping.
Catched when compiling `netmap` contract from nspcc-dev/neofs-contract.

Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>

pkg/compiler/codegen.go

@@ -555,9 +555,11 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 				vs := spec.(*ast.ValueSpec)
 				for i := range vs.Names {
 					obj := c.currPkg.Types.Scope().Lookup(vs.Names[i].Name)
-					c.constMap[c.getIdentName("", vs.Names[i].Name)] = types.TypeAndValue{
-						Type:  obj.Type(),
-						Value: obj.(*types.Const).Val(),
+					if obj != nil { // can be nil if unused
+						c.constMap[c.getIdentName("", vs.Names[i].Name)] = types.TypeAndValue{
+							Type:  obj.Type(),
+							Value: obj.(*types.Const).Val(),
+						}
 					}
 				}
 			}

pkg/compiler/global_test.go

@@ -12,6 +12,18 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestUnusedGlobal(t *testing.T) {
+	src := `package foo
+	const (
+		_ int = iota
+		a
+	)
+	func Main() int {
+		return 1
+	}`
+	eval(t, src, big.NewInt(1))
+}
+
 func TestChangeGlobal(t *testing.T) {
 	src := `package foo
 	var a int