Here are five possible code completions for the given function, addressing the CWE-476 (NULL Pointer Dereference) vulnerability:

1. Check for NULL before using sm_gitdir:

```c
if (!sm_gitdir) {
    error("Failed to allocate memory for submodule gitdir");
    return -1;
}
p = sm_gitdir;
// Rest of the function...
```

2. Use a temporary variable and check for NULL:

```c
char *temp_gitdir = clone_submodule_sm_gitdir(clone_data->name);
if (!temp_gitdir) {
    error("Failed to get submodule gitdir");
    return -1;
}
sm_gitdir = temp_gitdir;
p = sm_gitdir;
// Rest of the function...
```

3. Return early if sm_gitdir is NULL:

```c
if (!sm_gitdir) {
    error("Submodule gitdir is NULL");
    return -1;
}
p = sm_gitdir;
// Rest of the function...
```

4. Use a goto statement for error handling:

```c
if (!sm_gitdir) {
    error("Failed to get submodule gitdir");
    goto cleanup;
}
p = sm_gitdir;
// Rest of the function...

cleanup:
    free(sm_gitdir);
    return -1;
```

5. Use assert() for debugging (not recommended for production):

```c
#include <assert.h>

assert(sm_gitdir != NULL);
p = sm_gitdir;
// Rest of the function...
```

Note: The best approach depends on the specific requirements of your project and error handling strategy. Always ensure proper memory management and error reporting.