Root Cause Analysis: Looking at the given vulnerable code line and its context, the CWE-476 (NULL Pointer Dereference) vulnerability exists in the code because the `addr` pointer is being dereferenced at line 7 without being checked for NULL. The `hash_lookup` function at line 6 may return a NULL pointer if the key is not found in the hash table. If `addr` is NULL, dereferencing it at line 7 will result in a NULL pointer dereference, causing a crash or undefined behavior.

To mitigate this vulnerability, the code should check if `addr` is not NULL before dereferencing it, like this:

```c
if (addr != NULL) {
    addr->refcnt--;
    if (addr->refcnt == 0) {
        hash_release(bgp_address_hash, addr);
        XFREE(MTYPE_BGP_ADDR, addr);
    }
}
```