Q: Given the following code slice:
```
1 static void
2 setImage1(const uint8* br, uint32 rw, uint32 rh)
4     int step = rh;
5     int limit = tnh;
6     int err = 0;
10     uint32 dy;
11     for (dy = 0; dy < tnh; dy++) {
16 	err += step;
17 	while (err >= limit) {
18 	    err -= limit;
20 	    if (err >= limit)
21               rows[nrows++] = br + bpr*sy;```
which has a CWE-787 vulnerability at line:
```
21 		rows[nrows++] = br + bpr*sy;
```
Please analyze the root cause of the vulnerability.