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.

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