To generate the patch for the given code slice with a CWE-787 vulnerability, we can follow these reasoning steps:

### Step 1: Root Cause Analysis

- 1: Validate `strncpy` function usage:
  - The `strncpy` function is used to copy a string from `pszFileName` to `szFileName`.
  - However, the order of the arguments seems incorrect. The correct order should be `destination`, `source`, and `length`.

- 2: Argument Order:
  - In the given code, `sizeof(szFileName) - 5` is passed as the second argument, which is supposed to be the `source` string.
  - The `source` string `pszFileName` is passed as the third argument, which is supposed to be the `length`.

- 3: Potential Vulnerability:
  - The incorrect order of arguments can lead to undefined behavior, potentially causing a buffer overflow or other security vulnerabilities.
  - The `strncpy` function will attempt to copy `sizeof(szFileName) - 5` bytes from `pszFileName` to `szFileName`, but since `pszFileName` is not a valid length, this can result in writing past the end of the `szFileName` buffer.

### Conclusion:
The vulnerable behavior in this code is due to the incorrect order of arguments in the `strncpy` function. This can lead to undefined behavior and potential security vulnerabilities. To fix this, the correct order of arguments should be used.

### Step 2: Fixing Strategy

- A common fixing strategy is to correct the order of arguments in the `strncpy` function.
- The correct order should be `destination`, `source`, and `length`.
- Therefore, the patch should swap the second and third arguments, ensuring that the `source` string is copied to the `destination` buffer with the correct `length`.

The resulting patch is:
```
<         strncpy(szFileName, sizeof(szFileName) - 5, pszFileName);
---
>         strncpy(szFileName, pszFileName, sizeof(szFileName) - 5);
```
This patch corrects the order of arguments in the `strncpy` function, preventing potential buffer overflow and security vulnerabilities.