compiler: fix a bug with assignment to underscore

When using underscore it does not appear in the list
of local variables, so it can't be assigned.
In this commit the value is dropped.

pkg/compiler/codegen.go

@@ -253,9 +253,13 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 						ast.Walk(c, n.Rhs[i])
 					}
 
+					if t.Name == "_" {
+						emitOpcode(c.prog.BinWriter, opcode.DROP)
+					} else {
 						l := c.scope.loadLocal(t.Name)
 						c.emitStoreLocal(l)
 					}
+				}
 
 			case *ast.SelectorExpr:
 				switch expr := t.X.(type) {

pkg/compiler/return_test.go

@@ -37,6 +37,21 @@ func TestMultipleReturn2(t *testing.T) {
 	eval(t, src, big.NewInt(9))
 }
 
+func TestMultipleReturnUnderscore(t *testing.T) {
+	src := `
+		package hello
+		func f3() (int, int, int) {
+			return 5, 6, 7
+		}
+
+		func Main() int {
+			a, _, c := f3()
+			return a+c
+		}
+	`
+	eval(t, src, big.NewInt(12))
+}
+
 func TestMultipleReturnWithArg(t *testing.T) {
 	src := `
 		package hello