[Bug-tar] Bug in --sparse mode if file shrinks while tar is reading it

[selivan]

This bug was found by Chris Siebenmann. Here is a detailed post about it: 
https://utcc.utoronto.ca/~cks/space/blog/sysadmin/TarFindingTruncateBug

I did not find this bug, I am just reporting it.

Steps to reproduce:

- create a large sparse file
- start tar so that it archives the file
- while it works on the file, use ftruncate() to reduce file size.
- tar continues running forever generating infinite output

Probable location of the bug: sparse_dump_region() function in sparse.c