C++: Add test with missing flow.

cpp/ql/test/library-tests/dataflow/fields/simple.cpp

@@ -120,4 +120,20 @@ void post_update_to_phi_input(bool b)
   sink(a.i); // $ ast,ir
 }
 
-} // namespace Simple
+void write_to_param(int* p) {
+    *p = user_input();
+}
+
+void alias_with_fields(bool b) {
+    A a;
+    int* q;
+    if(b) {
+        q = &a.i;
+    } else {
+        q = nullptr;
+    }
+    write_to_param(q);
+    sink(a.i); // $ MISSING: ast,ir
+}
+
+} // namespace Simple