The CWE-787 vulnerability refers to "Out-of-bounds Write," which occurs when the software writes data past the end, or before the beginning, of the intended buffer. This can lead to corruption of data, crash of the system, or code execution vulnerabilities.

In the provided code, the vulnerability at line 57, `cp += es;`, is due to the potential for the pointer `cp` to move beyond the allocated memory of `tf_buf`. Let's analyze the root cause:

1. **Buffer Allocation**: The buffer `tf_buf` is allocated at line 11 with a size of `tf_bytesperrow`. This size is intended to hold one row of image data.

2. **Pointer Incrementation**: Within the loop starting at line 48, `cp` is a pointer that initially points to the start of `tf_buf` and is used to iterate over the data in `tf_buf`. The pointer `cp` is incremented several times within the loop:
   - At lines 52-55, `cp` is incremented by 1 for each case in the switch statement. The switch statement lacks `break` statements, causing fall-through behavior where `cp` is incremented multiple times (up to 4 times) per loop iteration depending on the value of `nc`.
   - At line 57, `cp` is further incremented by `es`, where `es` is calculated as `samplesperpixel - nc` at line 6.

3. **Potential Overrun**: The combination of increments (both in the switch statement and by `es`) can cause `cp` to exceed the bounds of `tf_buf`. This is particularly risky because:
   - The switch statement's fall-through behavior can lead to more increments than intended.
   - The value of `es` might be large enough to push `cp` beyond the buffer, especially if `samplesperpixel` is significantly larger than `nc`.

4. **Lack of Boundary Check**: There is no explicit boundary check before incrementing `cp` to ensure that it does not go past the end of `tf_buf`. This absence of boundary checking before pointer arithmetic is a common cause of buffer overflows.

5. **Incorrect Assumption**: The code might be operating under the assumption that the increments will always stay within bounds, which might not hold true for all values of `nc`, `samplesperpixel`, and the actual content of the TIFF file being processed.

To mitigate this vulnerability, the code should include checks to ensure that `cp` does not exceed `tf_buf + tf_bytesperrow` before performing any increments or operations that write to the buffer pointed by `cp`. Additionally, introducing `break` statements in the switch-case structure or rethinking its logic to prevent unintended multiple increments per loop iteration would be crucial.