Q: Given the following code slice:
```
  1 void
  2 PSDataColorContig(FILE* fd, TIFF* tif, uint32 w, uint32 h, int nc)
  3 {
  4         uint32 row;
  5         int breaklen = MAXLINE;
  6         int es = samplesperpixel - nc;
  7         tsize_t cc;
  8         unsigned char *tf_buf;
  9         unsigned char *cp, c;
 10         (void) w;
 11         tf_buf = (unsigned char *) _TIFFmalloc(tf_bytesperrow);
 12         if (tf_buf == NULL) {
 13                 TIFFError(filename, "No space for scanline buffer");
 14                 return;
 15         }
 16         for (row = 0; row < h; row++) {
 17                 if (TIFFReadScanline(tif, tf_buf, row, 0) < 0)
 18                         break;
 19                 cp = tf_buf;
 20                 /*
 21                  * for 16 bits, the two bytes must be most significant
 22                  * byte first
 23                  */
 24                 if (bitspersample == 16 && !HOST_BIGENDIAN) {
 25                         PS_FlipBytes(cp, tf_bytesperrow);
 26                 }
 27                 if (alpha) {
 28                         int adjust;
 29                         cc = 0;
 30                         for (; cc < tf_bytesperrow; cc += samplesperpixel) {
 31                                 DOBREAK(breaklen, nc, fd);
 32                                 /*
 33                                  * For images with alpha, matte against
 34                                  * a white background; i.e.
 35                                  *    Cback * (1 - Aimage)
 36                                  * where Cback = 1.
 37                                  */
 38                                 adjust = 255 - cp[nc];
 39                                 switch (nc) {
 40                                 case 4: c = *cp++ + adjust; PUTHEX(c,fd);
 41                                 case 3: c = *cp++ + adjust; PUTHEX(c,fd);
 42                                 case 2: c = *cp++ + adjust; PUTHEX(c,fd);
 43                                 case 1: c = *cp++ + adjust; PUTHEX(c,fd);
 44                                 }
 45                                 cp += es;
 46                         }
 47                 } else {
 48                         cc = 0;
 49                         for (; cc < tf_bytesperrow; cc += samplesperpixel) {
 50                                 DOBREAK(breaklen, nc, fd);
 51                                 switch (nc) {
 52                                 case 4: c = *cp++; PUTHEX(c,fd);
 53                                 case 3: c = *cp++; PUTHEX(c,fd);
 54                                 case 2: c = *cp++; PUTHEX(c,fd);
 55                                 case 1: c = *cp++; PUTHEX(c,fd);
 56                                 }
 57                                 cp += es;
 58                         }
 59                 }
 60         }
 61         _TIFFfree((char *) tf_buf);
 62 }```
which has a CWE-787 vulnerability at line:
```
57 				cp += es;
```
Please analyze the root cause of the vulnerability.