Here are five possible code completions for the given function, addressing the CWE-787 vulnerability (Out-of-bounds Write):

1.
```c
  length = INPUT_2BYTES(cinfo);
  n = INPUT_BYTE(cinfo);

  if (length != (n * 2 + 6) || n < 1 || n > MAX_COMPS_IN_SCAN)
    ERREXIT(cinfo, JERR_BAD_LENGTH);

  cinfo->comps_in_scan = n;

  for (i = 0; i < n; i++) {
    cc = INPUT_BYTE(cinfo);
    c = INPUT_BYTE(cinfo);
    for (ci = 0; ci < cinfo->num_components; ci++) {
      if (cc == cinfo->comp_info[ci].component_id)
        break;
    }
    if (ci == cinfo->num_components)
      ERREXIT1(cinfo, JERR_BAD_COMPONENT_ID, cc);
    compptr = &cinfo->comp_info[ci];
    cinfo->cur_comp_info[i] = compptr;
    compptr->dc_tbl_no = (c >> 4) & 15;
    compptr->ac_tbl_no = (c     ) & 15;
  }

  cinfo->Ss = INPUT_BYTE(cinfo);
  cinfo->Se = INPUT_BYTE(cinfo);
  cinfo->Ah = INPUT_BYTE(cinfo) >> 4;
  cinfo->Al = INPUT_BYTE(cinfo) & 15;
}
```

2.
```c
  length = INPUT_2BYTES(cinfo);
  n = INPUT_BYTE(cinfo);

  if (length != (n * 2 + 6) || n < 1 || n > MAX_COMPS_IN_SCAN)
    ERREXIT(cinfo, JERR_BAD_LENGTH);

  cinfo->comps_in_scan = n;

  for (i = 0; i < n; i++) {
    cc = INPUT_BYTE(cinfo);
    c = INPUT_BYTE(cinfo);
    for (ci = 0; ci < cinfo->num_components && ci < MAX_COMPONENTS; ci++) {
      if (cc == cinfo->comp_info[ci].component_id)
        break;
    }
    if (ci == cinfo->num_components || ci == MAX_COMPONENTS)
      ERREXIT1(cinfo, JERR_BAD_COMPONENT_ID, cc);
    compptr = &cinfo->comp_info[ci];
    if (i < MAX_COMPS_IN_SCAN)
      cinfo->cur_comp_info[i] = compptr;
    compptr->dc_tbl_no = (c >> 4) & 15;
    compptr->ac_tbl_no = (c     ) & 15;
  }

  cinfo->Ss = INPUT_BYTE(cinfo);
  cinfo->Se = INPUT_BYTE(cinfo);
  cinfo->Ah = INPUT_BYTE(cinfo) >> 4;
  cinfo->Al = INPUT_BYTE(cinfo) & 15;
}
```

3.
```c
  length = INPUT_2BYTES(cinfo);
  n = INPUT_BYTE(cinfo);

  if (length != (n * 2 + 6) || n < 1 || n > MAX_COMPS_IN_SCAN)
    ERREXIT(cinfo, JERR_BAD_LENGTH);

  cinfo->comps_in_scan = n;

  for (i = 0; i < n && i < MAX_COMPS_IN_SCAN; i++) {
    cc = INPUT_BYTE(cinfo);
    c = INPUT_BYTE(cinfo);
    for (ci = 0; ci < cinfo->num_components; ci++) {
      if (cc == cinfo->comp_info[ci].component_id)
        break;
    }
    if (ci == cinfo->num_components)
      ERREXIT1(cinfo, JERR_BAD_COMPONENT_ID, cc);
    compptr = &cinfo->comp_info[ci];
    cinfo->cur_comp_info[i] = compptr;
    compptr->dc_tbl_no = (c >> 4) & 15;
    compptr->ac_tbl_no = (c     ) & 15;
  }

  cinfo->Ss = INPUT_BYTE(cinfo);
  cinfo->Se = INPUT_BYTE(cinfo);
  cinfo->Ah = INPUT_BYTE(cinfo) >> 4;
  cinfo->Al = INPUT_BYTE(cinfo) & 15;
}
```

4.
```c
  length = INPUT_2BYTES(cinfo);
  n = INPUT_BYTE(cinfo);

  if (length != (n * 2 + 6) || n < 1 || n > MAX_COMPS_IN_SCAN)
    ERREXIT(cinfo, JERR_BAD_LENGTH);

  cinfo->comps_in_scan = (n > MAX_COMPS_IN_SCAN) ? MAX_COMPS_IN_SCAN : n;

  for (i = 0; i < cinfo->comps_in_scan; i++) {
    cc = INPUT_BYTE(cinfo);
    c = INPUT_BYTE(cinfo);
    for (ci = 0; ci < cinfo->num_components; ci++) {
      if (cc == cinfo->comp_info[ci].component_id)
        break;
    }
    if (ci == cinfo->num_components)
      ERREXIT1(cinfo, JERR_BAD_COMPONENT_ID, cc);
    compptr = &cinfo->comp_info[ci];
    cinfo->cur_comp_info[i] = compptr;
    compptr->dc_tbl_no = (c >> 4) & 15;
    compptr->ac_tbl_no = (c     ) & 15;
  }

  cinfo->Ss = INPUT_BYTE(cinfo);
  cinfo->Se = INPUT_BYTE(cinfo);
  cinfo->Ah = INPUT_BYTE(cinfo) >> 4;
  cinfo->Al = INPUT_BYTE(cinfo) & 15;
}
```

5.
```c
  length = INPUT_2BYTES(cinfo);
  n = INPUT_BYTE(cinfo);

  if (length != (n * 2 + 6) || n < 1 || n > MAX_COMPS_IN_SCAN)
    ERREXIT(cinfo, JERR_BAD_LENGTH);

  cinfo->comps_in_scan = n;

  for (i = 0; i < n; i++) {
    cc = INPUT_BYTE(cinfo);
    c = INPUT_BYTE(cinfo);
    for (ci = 0; ci < cinfo->num_components; ci++) {
      if (cc == cinfo->comp_info[ci].component_id)
        break;
    }
    if (ci == cinfo->num_components)
      ERREXIT1(cinfo, JERR_BAD_COMPONENT_ID, cc);
    compptr = &cinfo->comp_info[ci];
    if (i < sizeof(cinfo->cur_comp_info) / sizeof(cinfo->cur_comp_info[0]))
      cinfo->cur_comp_info[i] = compptr;
    compptr->dc_tbl_no = (c >> 4) & 15;
    compptr->ac_tbl_no = (c     ) & 15;
  }

  cinfo->Ss = INPUT_BYTE(cinfo);
  cinfo->Se = INPUT_BYTE(cinfo);
  cinfo->Ah = INPUT_BYTE(cinfo) >> 4;
  cinfo->Al = INPUT_BYTE(cinfo)