Here are five possible code completions for the given function:

**Option 1: Basic Row Retrieval**

```c
if (image_ptr == NULL) {
    ERREXIT(cinfo, JERR_OUT_OF_MEMORY);
}
inptr = image_ptr[0];
outptr = source->output_buf;
for (col = cinfo->image_width; col > 0; col--) {
    *outptr++ = GETJSAMPLE(*inptr++);
}
return source->output_buf;
```

**Option 2: Row Retrieval with Error Handling**

```c
if (image_ptr == NULL) {
    ERREXIT(cinfo, JERR_OUT_OF_MEMORY);
}
inptr = image_ptr[0];
outptr = source->output_buf;
for (col = cinfo->image_width; col > 0; col--) {
    if (col >= cinfo->image_width) {
        ERREXIT(cinfo, JERR_WIDTH_OVERFLOW);
    }
    *outptr++ = GETJSAMPLE(*inptr++);
}
return source->output_buf;
```

**Option 3: Row Retrieval with Colormap Indexing**

```c
if (image_ptr == NULL) {
    ERREXIT(cinfo, JERR_OUT_OF_MEMORY);
}
inptr = image_ptr[0];
outptr = source->output_buf;
for (col = cinfo->image_width; col > 0; col--) {
    t = *inptr++;
    if (t < 0 || t >= cmaplen) {
        ERREXIT(cinfo, JERR_OUT_OF_RANGE);
    }
    *outptr++ = colormap[t];
}
return source->output_buf;
```

**Option 4: Row Retrieval with Bounds Checking**

```c
if (image_ptr == NULL) {
    ERREXIT(cinfo, JERR_OUT_OF_MEMORY);
}
inptr = image_ptr[0];
outptr = source->output_buf;
for (col = cinfo->image_width; col > 0; col--) {
    if (inptr >= image_ptr[0] + cinfo->image_width) {
        ERREXIT(cinfo, JERR_WIDTH_OVERFLOW);
    }
    *outptr++ = GETJSAMPLE(*inptr++);
}
return source->output_buf;
```

**Option 5: Row Retrieval with Colormap Indexing and Bounds Checking**

```c
if (image_ptr == NULL) {
    ERREXIT(cinfo, JERR_OUT_OF_MEMORY);
}
inptr = image_ptr[0];
outptr = source->output_buf;
for (col = cinfo->image_width; col > 0; col--) {
    if (inptr >= image_ptr[0] + cinfo->image_width) {
        ERREXIT(cinfo, JERR_WIDTH_OVERFLOW);
    }
    t = *inptr++;
    if (t < 0 || t >= cmaplen) {
        ERREXIT(cinfo, JERR_OUT_OF_RANGE);
    }
    *outptr++ = colormap[t];
}
return source->output_buf;
```