server crash.

The filesort implementation has an optimization for subquery execution which
consists of reusing previously allocated buffers. In particular the call to
the read_buffpek_from_file function might be skipped when a big enough buffer
for buffer descriptors (buffpeks) is already allocated. Beside allocating
memory for buffpeks this function fills allocated buffer with data read from
disk. Skipping it might led to using an arbitrary memory as fields' data and
finally to a crash.

Now the read_buffpek_from_file function is always called. It allocates
new buffer only when necessary, but always fill it with correct data.


sql/filesort.cc:
  Bug#33675: Usage of an uninitialized memory by filesort in a subquery caused
  server crash.Now the read_buffpek_from_file function is always called. It allocates
  new buffer only when necessary, but always fill it with correct data.
mysql-test/r/subselect.result:
  Added a test case for the bug#33675: Usage of an uninitialized memory by
  filesort in a subquery caused server crash.
mysql-test/t/subselect.test:
  Added a test case for the bug#33675: Usage of an uninitialized memory by
  filesort in a subquery caused server crash.