The CWE-787 vulnerability at line 57 is caused by a potential out-of-bounds write due to an incorrect pointer arithmetic operation.

Here's the root cause analysis:

1.  The `cp` pointer is initialized to point to the beginning of the `tf_buf` array, which is allocated to hold a single scanline of the image.
2.  The `es` variable is calculated as `samplesperpixel - nc`, where `samplesperpixel` is the total number of samples per pixel in the image, and `nc` is the number of color components being processed.
3.  In the loop, the `cp` pointer is incremented by `es` bytes after processing each pixel. This is intended to skip over the remaining samples in the pixel that are not being processed.
4.  However, if `es` is not correctly calculated or if the `samplesperpixel` value is not accurate, the `cp` pointer may be incremented beyond the bounds of the `tf_buf` array.
5.  When `cp` is incremented beyond the bounds of the array, the subsequent writes to `cp` (in the `PUTHEX` macro) will write to memory outside the allocated buffer, causing an out-of-bounds write.

The root cause of the vulnerability is the incorrect calculation of the `es` value or the `samplesperpixel` value, which can lead to an incorrect pointer arithmetic operation.

To fix this vulnerability, you should:

*   Verify that the `samplesperpixel` value is accurate and correctly calculated.
*   Ensure that the `es` value is correctly calculated and does not exceed the bounds of the `tf_buf` array.
*   Add bounds checking to prevent the `cp` pointer from being incremented beyond the bounds of the array.

Here's an example of how you can add bounds checking:
```c
if (cp + es > tf_buf + tf_bytesperrow) {
    // Handle error: out-of-bounds write
    TIFFError(filename, "Out-of-bounds write detected");
    return;
}
cp += es;
```
By adding bounds checking, you can prevent the out-of-bounds write and ensure that the code is secure.