bzip2: fix bug with duplicate read of CRC32 when it overlaps end of buffer and 'bzip2-done is thrown.

[vseloved]

This bug is quite rare and usually manifests only on large bzip2 files (several 100s of Megabytes in size). For instance, it can be reproduced while processing this part of wikipedia dump with the following simple script:

(with-open-file (f "enwiki-20170720-pages-meta-current12.xml-p3926863p5040436.bz2"
                 :element-type 'unsigned-byte)
   (let* ((in (chipz:make-decompressing-stream :bzip2 f))
          (in (flex:make-flexi-stream in :external-format :utf-8)))
      (loop :for line := (read-line in nil) :while line)))

At block 383 the invalid-bzip2-data error will be signalled.

We also had a conversation about the same bug some time ago in the issues (which are not accessible now, unfortunately).