The root cause of the CWE-787 (Out-of-bounds Write) vulnerability at line 35 is an incorrect boundary check that fails to prevent potential buffer overflow.

Let's break down the issue:

1. The vulnerability occurs in the else-if branch starting at line 29.

2. In this branch, the code first advances the 'frame' pointer by 'v' bytes (line 33).

3. Then, it attempts to write 4 bytes (2 pairs of 2 bytes each) starting from the new 'frame' position (lines 37-42).

4. The check at line 35 is intended to ensure there's enough space for these writes, but it's inadequate:

   ```c
   if (frame_end - frame < width + 3)
   ```

5. This check has two main problems:
   a) It uses 'width + 3' instead of 4, which is the actual number of bytes about to be written.
   b) It doesn't account for the possibility that 'frame' might already be very close to or even beyond 'frame_end' after the advancement in line 33.

6. As a result, even if this check passes, it doesn't guarantee that the subsequent writes will be within bounds.

7. If 'frame' is already past 'frame_end' or if there's less than 4 bytes of space left, the writes on lines 37-42 will cause an out-of-bounds write, potentially overwriting memory that doesn't belong to the buffer.

To fix this vulnerability, the check should be modified to ensure that there's always at least 4 bytes of space left in the buffer before performing the writes. Additionally, it should verify that 'frame' hasn't already moved past 'frame_end' after the advancement in line 33.