The test case was setting aria_sort_buffer_size to MAX_ULONGLONG-1
which was not handled gracefully by my_malloc() or safemalloc().
Fixed by ensuring that the malloc functions returns 0 if the size
is too big.
I also added some protection to Aria repair:
- Limit sort_buffer_size to 16G (after that a bigger sort buffer will
  not help that much anyway)
- Limit sort_buffer_size also according to sort file size. This will
  help by not allocating less memory if someone sets the buffer size too
  high.