Root Cause Analysis: The CWE-787 (Out-of-bounds Write) vulnerability exists in the given code because there is no proper bounds checking before writing to the `frame` array. Specifically, at line 40, the code writes two bytes to `frame[0]`, `frame[1]`, `frame[width]`, and `frame[width + 1]` without ensuring that `frame` has enough space to accommodate these writes.

The vulnerability occurs because the code assumes that `frame_end - frame` is always greater than or equal to `width + 3`, which is checked at line 35. However, this check is not sufficient to guarantee that the writes at line 40 are safe, as it does not account for the possibility that `frame` may have already been advanced beyond the end of the buffer due to previous writes.

To mitigate this vulnerability, the code should perform proper bounds checking before each write operation to ensure that the write does not exceed the bounds of the `frame` buffer.