The provided code snippet does not have a CWE-787 (Out-of-bounds Write) vulnerability at the specified line. The vulnerability actually lies within the loop and the use of `w_i` as an index. Here's a breakdown:

**Vulnerable Behavior:**

1. **Input:** The function takes `data_len` as an input, representing the length of the data to be processed. 

2. **Loop Iteration:** The loop iterates using `i` with a step of 4 bytes (`i += 4`) but aims to process word-by-word using `w_i` as the word index.

3. **Potential Out-of-Bounds Access:** The calculation `int w_i = i / 4;` aims to convert the byte index `i` to a word index `w_i`. However, this calculation doesn't account for the possibility that `data_len` might not be a multiple of 4.

4. **Vulnerable Access:** Inside the loop, `src[w_i]`, `dest[w_i]`, and the `bootloader_sha256_data` call all use `w_i` as an index. If `data_len` is not a multiple of 4, on the last iteration, `w_i` can exceed the valid index range for `src` and `dest`, leading to an out-of-bounds access.

**Example:**

Let's say `data_len` is 7 bytes. The loop will iterate twice:

* **Iteration 1:** `i = 0`, `w_i = 0`. Accesses `src[0]` and `dest[0]` - within bounds.
* **Iteration 2:** `i = 4`, `w_i = 1`.  Accesses `src[1]`, which is out of bounds as `src` only has valid data up to index 0 for a 7-byte length.

**Conclusion:**

The vulnerability arises because the loop doesn't properly handle the case when `data_len` is not a multiple of 4. This can lead to out-of-bounds accesses when using `w_i` as an index.

**Fix:**

To fix this, the code should ensure that the loop only iterates within the valid bounds of the data. One way to do this is to calculate the number of words to process before the loop and use a separate index for word access:

```c
    size_t num_words = data_len / 4; // Calculate number of words
    for (size_t i = 0; i < num_words; i++) { // Iterate over words
        uint32_t w = src[i]; 
        // ... rest of the code using i as the word index
    }

    // Handle any remaining bytes (if data_len is not a multiple of 4)
    if (data_len % 4 != 0) {
        // ... process the remaining bytes carefully
    }
```

This fix ensures that the code doesn't access memory outside the allocated buffer, preventing the out-of-bounds write vulnerability.
