Trivial Fix: use palloc_array/repalloc_array for BufFile file arrays

Hi Hackers,

I noticed this error while working on [1].

In BufFile, the fields is claimed as an array:

```

struct BufFile
{

    File *files; /* palloc'd array with numFiles entries */

```

However, it’s allocated by palloc_object():

```

file->files = palloc_object(File);

```

And reallocated by repalloc():

```

    file->files = (File *) repalloc(file->files,
        (file->numFiles + 1) * sizeof(File));

```

This trivial patch just changes to use palloc_array/repalloc_array, which makes the intent clearer.

--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/