```c
static int
process_cu_tu_index (struct dwarf_section *section, int do_display)
{
  unsigned char *phdr = section->start;
  unsigned char *limit = phdr + section->size;
  unsigned char *phash;
  unsigned char *pindex;
  unsigned char *ppool;
  unsigned int version;
  unsigned int ncols = 0;
  unsigned int nused;
  unsigned int nslots;
  unsigned int i;
  unsigned int j;
  dwarf_vma signature_high;
  dwarf_vma signature_low;
  char buf[64];

  // 1. Validate header size before accessing version:
  if (limit - phdr < 5) 
  {
    fprintf (stderr, "Error: CU/TU index section too small.\n");
    return -1;
  }
  version = *phdr++;

  // 2. Check version and proceed accordingly:
  if (version == 1) {
    // ... (process version 1 format)
  } else if (version == 2) {
    // ... (process version 2 format)
  } else {
    fprintf (stderr, "Error: Unknown CU/TU index version: %u\n", version);
    return -1; 
  }

  // 3. Validate data size before accessing any further fields:
  if (limit - phdr < /* size of remaining header fields based on version */) {
    fprintf (stderr, "Error: CU/TU index section truncated.\n");
    return -1;
  }

  // 4. Check for buffer overflows when processing variable-length data:
  // ... (e.g., when reading strings or arrays)

  // 5. Handle potential errors during data processing:
  // ... (e.g., check return values of functions)

  // ... (rest of the function implementation)
}
```

**Explanation:**

1. **Validate header size:** Before accessing `version`, we need to ensure that the header has at least 5 bytes. Otherwise, accessing `*phdr++` would lead to a buffer overflow.
2. **Version check:** The code should handle different versions of the CU/TU index format. Unknown versions should be treated as errors.
3. **Data size validation:** After reading the version, we need to validate the remaining header size based on the expected format for that version. Accessing fields beyond the actual data size would lead to a buffer overflow.
4. **Buffer overflow prevention:** When processing variable-length data like strings or arrays, ensure that the code doesn't read beyond the allocated buffer size (`buf` in this case).
5. **Error handling:** Implement proper error handling for potential issues like invalid data formats or failures in subsequent processing steps.

Remember to replace the comments with actual code based on the specific format and processing logic of the CU/TU index section. 
