rar5: Fix random initial offset if using archive_read_data_into_fd

rar5: Fix random initial offset if using archive_read_data_into_fd

archive_read_data_into_fd passes a pointer to an uninitialized
variable as an output 'offset' argument into archive_read_data_block
function, and expects that this variable will always be initialized
inside of it.

Like this:
  size_t size;
  int64_t offset;
  archive_read_data_block(a, &buf, &size, &offset);
  /* some work with offset here */

But rar5 implementation of archive_read_data_block function leaves the
'offset' argument uninitialized in one code path (if file is
compressed and there are no uncompressed pending data blocks).

As a result, archive_read_data_info_fd function is using an
uninitialized variable as an initial offset of an output file. And in
most cases it causes an appending sparse block of a random size at the
beginning of the output file.