Step 1: 
A: The first step is to add a pre-conditional check to ensure that `func_obj_p` is not NULL and it does not represent an environment object (lexical environment). This can be achieved by adding the following line at the beginning of the function:

```c
if (func_obj_p == NULL || ecma_is_lexical_environment (func_obj_p)) {
    JERRY_ASSERT (false);
}
```

The second step is to add a post-conditional check to ensure that the function's return value is valid. This can be achieved by adding the following line at the end of the function:

```c
if (!ecma_is_value_valid_for_routine (ret_value)) {
    JERRY_ASSERT (false);
}
```

Here, `ecma_is_value_valid_for_routine` is a hypothetical function that checks if the returned value is valid for routine use. If it is not valid, the function should assert false to indicate an error.

By adding these two checks, we ensure that the function behaves correctly and does not produce invalid results.