LibJS: Don't assume [[GetOwnPropertyDescriptor]] always succeeds

It can fail if we're talking to a badly-behaved proxy when enumerating
object properties for iteration.

Libraries/LibJS/Bytecode/Interpreter.cpp

@@ -1732,6 +1732,8 @@ inline ThrowCompletionOr<Object*> get_object_property_iterator(VM& vm, Value val
                 continue;
 
             auto descriptor = TRY(object_to_check->internal_get_own_property(property_key));
+            if (!descriptor.has_value())
+                continue;
             if (!*descriptor->enumerable)
                 non_enumerable_properties.set(move(property_key));
             else

Libraries/LibJS/Tests/builtins/Proxy/iterate-over-proxy.js

@@ -0,0 +1,12 @@
+test("iterate over bogus proxy", () => {
+    expect(() => {
+        let proxy = new Proxy([123], {
+            getOwnPropertyDescriptor: function (p) {
+                return undefined;
+            },
+        });
+
+        for (const p in proxy) {
+        }
+    }).toThrow();
+});