To generate the patch for the CWE-787 (Out-of-bounds write) vulnerability at line `72 buf[buflen] = '\0';`, we need to follow a structured approach to identify the root cause and then apply a suitable fix. Here are the reasoning steps:

### Step 1: Root Cause Analysis

1. **Understanding the Code Context:**
   - The function `write_syslog` is responsible for writing a log message to the system log.
   - The variable `len` holds the length of the input string `line`.
   - The loop processes the input string `line` in chunks of size `PG_SYSLOG_LIMIT`.

2. **Identifying the Vulnerable Behavior:**
   - The function `pg_mbcliplen` is used to determine the length of a multibyte character string that fits within a specified byte length.
   - The result of `pg_mbcliplen` is stored in `buflen`.
   - The line `buf[buflen] = '\0';` attempts to null-terminate the buffer `buf` at the position `buflen`.

3. **Potential Issue with `buflen`:**
   - If `pg_mbcliplen` returns a value less than or equal to 0, `buflen` could be 0 or negative.
   - Writing to `buf[0]` or `buf[-1]` would result in an out-of-bounds write, leading to undefined behavior and potential security vulnerabilities.

### Step 2: Fixing Strategy

1. **Adding a Check for `buflen`:**
   - To prevent the out-of-bounds write, we need to ensure that `buflen` is greater than 0 before attempting to write to `buf[buflen]`.
   - If `buflen` is less than or equal to 0, the function should return early to avoid any further processing that could lead to an out-of-bounds write.

2. **Implementing the Fix:**
   - Add a conditional check after calculating `buflen` to verify that it is greater than 0.
   - If `buflen` is less than or equal to 0, return from the function to prevent the out-of-bounds write.

### Conclusion

The patch ensures that the function does not attempt to write outside the bounds of the `buf` array by adding a check for `buflen`. If `buflen` is less than or equal to 0, the function returns early, thus preventing the out-of-bounds write.

### Patch Implementation

Here is the patch with the added check:

```diff
<             buflen = pg_mbcliplen(buf, l, l);
<             buf[buflen] = '\0';
---
>             buflen = pg_mbcliplen(buf, l, l);
>             if (buflen <= 0)
>             {
>                 return;
>             }
>             buf[buflen] = '\0';
```

### Reasoning Steps Summary

1. **Identify the root cause:** The out-of-bounds write occurs because `buflen` could be less than or equal to 0, leading to invalid indexing of the `buf` array.
2. **Determine the fix:** Add a check to ensure `buflen` is greater than 0 before writing to `buf[buflen]`.
3. **Implement the fix:** Modify the code to include the check and return early if `buflen` is less than or equal to 0.

By following these steps, we ensure that the code is safe from out-of-bounds writes, thus mitigating the CWE-787 vulnerability.