To generate the patch for the given code slice, we need to follow a structured approach to identify the root cause of the vulnerability and then apply an appropriate fix. Here are the reasoning steps:

### Step 1: Root Cause Analysis

1. **Understanding the Code:**
   - The function `copyIPv6IfDifferent` is designed to copy data from a source (`src`) to a destination (`dest`) if the source and destination are different.
   - The `memcpy` function is used to perform the copy operation, with the size of the data being `sizeof(struct in6_addr)`.

2. **Identifying the Vulnerability:**
   - The vulnerability in question is CWE-476: NULL Pointer Dereference.
   - A NULL pointer dereference occurs when the code attempts to read or write to a memory location that is pointed to by a NULL pointer.
   - In this code, the `memcpy` function is called without checking if `src` is NULL. If `src` is NULL, `memcpy` will attempt to read from a NULL pointer, leading to undefined behavior and potentially crashing the program.

3. **Potential Vulnerable Behavior:**
   - If `src` is NULL, the `memcpy` function will dereference a NULL pointer, causing a runtime error.
   - The current condition `if(dest != src)` only checks if `dest` and `src` are different but does not ensure that `src` is a valid (non-NULL) pointer.

### Step 2: Fixing Strategy

1. **Adding a NULL Check:**
   - To prevent the NULL pointer dereference, we need to ensure that `src` is not NULL before calling `memcpy`.
   - This can be achieved by modifying the condition to include a check for `src` being non-NULL.

2. **Implementing the Patch:**
   - The original condition `if(dest != src)` should be updated to `if(dest != src && src != NULL)`.
   - This ensures that the `memcpy` function is only called if `src` is a valid pointer and not NULL.

### Conclusion

The patch ensures that the `memcpy` function is only called when `src` is a valid, non-NULL pointer, thus preventing the NULL pointer dereference vulnerability.

### Final Patch

```diff
< 	if(dest != src) {
---
> 	if(dest != src && src != NULL) {
```

By following these reasoning steps, we can understand the root cause of the vulnerability and apply an appropriate fix to ensure the code is safe from NULL pointer dereference issues.