LibJS: Fix crash in bytecode generator on https://twinings.co.uk/

If the current block has already been terminated, we should just skip
creating a per-iteration environment.

Userland/Libraries/LibJS/Tests/for-loop-per-iteration-env-bug.js

@@ -0,0 +1,12 @@
+test("check that codegen doesn't crash", () => {
+    function func(x) {
+        expect(x()).toBe(0);
+    }
+
+    function go() {
+        for (let i = 0; ; ) {
+            func(() => i);
+            break;
+        }
+    }
+});