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

1. Bounds checking and early return:

```c
    if (length == 0 || caplen == 0) {
        ND_PRINT("Invalid packet length");
        return;
    }

    if (length > caplen) {
        length = caplen;
    }

    b = (u_char *)malloc(length);
    if (b == NULL) {
        ND_PRINT("Memory allocation failed");
        return;
    }

    for (i = 0, s = p, t = b; i < length; i++) {
        c = *s++;
        if (c == 0x7d) {
            if (i + 1 >= length) {
                break;
            }
            c = *s++ ^ 0x20;
            i++;
        }
        *t++ = c;
    }

    proto = *b;
    if (length > 1)
        proto = (proto << 8) | b[1];

    ND_PRINT("%s ", tok2str(ppptype2str, "unknown PPP protocol (0x%04x)", proto));

    free(b);
```

2. Using a safer string copy function:

```c
    size_t safe_length = (length < caplen) ? length : caplen;
    
    b = (u_char *)malloc(safe_length);
    if (b == NULL) {
        ND_PRINT("Memory allocation failed");
        return;
    }

    memcpy(b, p, safe_length);

    for (i = 0, t = b; i < safe_length; i++) {
        if (b[i] == 0x7d) {
            if (i + 1 >= safe_length) {
                break;
            }
            b[i + 1] ^= 0x20;
            memmove(&b[i], &b[i + 1], safe_length - i - 1);
            safe_length--;
        }
    }

    proto = (safe_length > 0) ? b[0] : 0;
    if (safe_length > 1)
        proto = (proto << 8) | b[1];

    ND_PRINT("%s ", tok2str(ppptype2str, "unknown PPP protocol (0x%04x)", proto));

    free(b);
```

3. Using a fixed-size buffer with bounds checking:

```c
    #define MAX_BUFFER_SIZE 1024
    u_char buffer[MAX_BUFFER_SIZE];
    size_t safe_length = (length < caplen) ? length : caplen;
    safe_length = (safe_length < MAX_BUFFER_SIZE) ? safe_length : MAX_BUFFER_SIZE;

    memcpy(buffer, p, safe_length);

    for (i = 0, t = buffer; i < safe_length; i++) {
        if (buffer[i] == 0x7d) {
            if (i + 1 >= safe_length) {
                break;
            }
            buffer[i + 1] ^= 0x20;
            memmove(&buffer[i], &buffer[i + 1], safe_length - i - 1);
            safe_length--;
        }
    }

    proto = (safe_length > 0) ? buffer[0] : 0;
    if (safe_length > 1)
        proto = (proto << 8) | buffer[1];

    ND_PRINT("%s ", tok2str(ppptype2str, "unknown PPP protocol (0x%04x)", proto));
```

4. Using a two-pass approach to determine the exact buffer size needed:

```c
    size_t safe_length = (length < caplen) ? length : caplen;
    size_t decoded_length = 0;

    // First pass: count the number of escape characters
    for (i = 0; i < safe_length; i++) {
        if (p[i] == 0x7d) {
            i++;
        }
        decoded_length++;
    }

    b = (u_char *)malloc(decoded_length);
    if (b == NULL) {
        ND_PRINT("Memory allocation failed");
        return;
    }

    // Second pass: decode the data
    for (i = 0, t = b; i < safe_length; i++) {
        c = p[i];
        if (c == 0x7d) {
            if (i + 1 >= safe_length) {
                break;
            }
            c = p[++i] ^ 0x20;
        }
        *t++ = c;
    }

    proto = (decoded_length > 0) ? b[0] : 0;
    if (decoded_length > 1)
        proto = (proto << 8) | b[1];

    ND_PRINT("%s ", tok2str(ppptype2str, "unknown PPP protocol (0x%04x)", proto));

    free(b);
```

5. Using a custom safe copy function:

```c
    size_t safe_length = (length < caplen) ? length : caplen;

    b = (u_char *)malloc(safe_length);
    if (b == NULL) {
        ND_PRINT("Memory allocation failed");
        return;
    }

    size_t decoded_length = safe_copy_and_unescape(b, p, safe_length);

    proto = (decoded_length > 0) ? b[0] : 0;
    if (decoded_length > 1)
        proto = (proto << 8) | b[1];

    ND_PRINT("%s ", tok2str(ppptype2str, "unknown PPP protocol (0x%04x)", proto));

    free(b);
}

size_t safe_copy_and_unescape(u_char *dst, const u_char *src, size_t length) {
    size_t i, j;
    for (i = 0, j = 0; i < length; i++, j++) {
        if (src[i] == 0x7d) {
            if (i + 1 >= length) {
                break;
            }
            dst[j] = src[++i] ^ 0x20;
        } else {
            dst[j] = src[i];
        }
    }
    return j;
}
```

These completions address the CWE-787 vulnerability by ensuring proper bounds checking, using safer memory allocation and copying techniques, and handling edge cases appropriately.