The root cause of the CWE-476 (NULL Pointer Dereference) vulnerability in this code slice is:

The JERRY_ASSERT macro on line 7 attempts to dereference the func_obj_p pointer without first ensuring it is not NULL. While the assertion does check if func_obj_p is NULL, it does so in a compound condition that can lead to a NULL pointer dereference if func_obj_p is indeed NULL.

Specifically:

1. The assertion uses a logical AND (&&) operator to combine two conditions.
2. The first condition checks if func_obj_p is not NULL.
3. However, if func_obj_p is NULL, the second part of the condition (!ecma_is_lexical_environment(func_obj_p)) will still be evaluated due to the nature of the && operator.
4. This evaluation will attempt to dereference func_obj_p by passing it to the ecma_is_lexical_environment function, even though it's NULL.

This can lead to a NULL pointer dereference if func_obj_p is NULL, potentially causing a program crash or undefined behavior.

To fix this, the assertion should be split into two separate checks:

```c
JERRY_ASSERT (func_obj_p != NULL);
JERRY_ASSERT (!ecma_is_lexical_environment (func_obj_p));
```

This way, the second assertion will only be evaluated if func_obj_p is not NULL, preventing the potential NULL pointer dereference.