fix: handle empty case body (#244)

_test/if1.go

@@ -0,0 +1,11 @@
+package main
+
+func main() {
+	i := 1
+	if i > 0 {
+	}
+	println("bye")
+}
+
+// Output:
+// bye

_test/switch20.go

@@ -0,0 +1,16 @@
+package main
+
+func main() {
+	i := 1
+
+	switch i {
+	case 1:
+		// nothing to do
+	default:
+		println("not run")
+	}
+	println("bye")
+}
+
+// Output:
+// bye

interp/cfg.go

@@ -750,9 +750,13 @@ func (interp *Interpreter) cfg(root *node) ([]*node, error) {
 
 		case caseBody:
 			wireChild(n)
-			if typeSwichAssign(n) && len(n.child) > 1 {
+			switch {
+			case typeSwichAssign(n) && len(n.child) > 1:
 				n.start = n.child[1].start
-			} else {
+			case len(n.child) == 0:
+				// empty case body: jump to switch node (exit node)
+				n.start = n.anc.anc.anc
+			default:
 				n.start = n.child[0].start
 			}