I'll provide the reasoning steps for generating this patch following the two-step process:

Step 1. Root Cause Analysis:

1. Function parameters:
   - The function takes two void pointers: `dest` and `src`.
   - These pointers are not checked for NULL values before use.

2. Conditional check:
   - The existing check `if(dest != src)` only ensures that the source and destination are different.
   - It doesn't verify if either pointer is NULL.

3. Memory operation:
   - `memcpy` is used to copy data from `src` to `dest`.
   - If `src` is NULL, dereferencing it in `memcpy` will cause a null pointer dereference (CWE-476).

4. Potential vulnerability:
   - The vulnerability occurs when `src` is NULL, as it will be dereferenced in the `memcpy` call.
   - This can lead to a program crash or undefined behavior.

Conclusion:
The vulnerable behavior is caused by not checking if `src` is NULL before using it in `memcpy`. This can lead to a null pointer dereference, which is a CWE-476 vulnerability.

Step 2. Fixing Strategy:

To fix this vulnerability, we need to add a check to ensure that `src` is not NULL before performing the `memcpy` operation. A common fixing strategy is to add this check in the existing conditional statement.

Therefore, the patch should be:

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

This patch adds an additional condition to check if `src` is not NULL, preventing the null pointer dereference vulnerability. The `memcpy` operation will only be performed if both conditions are met: `dest` and `src` are different, and `src` is not NULL.