[Toybox] tar x broken with (some) sparse files

[enh]

haven't had time to investigate yet, but the the .tar.bz2 file at

https://android.googlesource.com/platform/system/update_engine/+/refs/heads/master/sample_images/

is interpreted differently by GNU tar and toybox tar. here's tvf:

$ tar tvf sample_images.tar.bz2
-rw-r----- ahassani/eng 1048576 2017-09-28 10:36 disk_ext2_1k.img
-rw-r----- ahassani/eng 4194304 2017-09-28 10:36 disk_ext2_4k.img
-rw-r----- ahassani/eng 4194304 2017-09-28 10:36 disk_ext2_4k_empty.img
-rw-r----- ahassani/eng 4194304 2017-09-28 10:37 disk_ext2_unittest.img
-rw-r----- ahassani/eng    4096 2017-09-28 10:37 disk_sqfs_empty.img
-rw-r----- ahassani/eng    4096 2017-09-28 10:37 disk_sqfs_default.img
$ toybox tar tvf sample_images.tar.bz2
-rw-r----- ahassani/eng     41472  2017-09-28 10:36 disk_ext2_1k.img
-rw-r----- ahassani/eng     41472  2017-09-28 10:36 disk_ext2_4k.img
-rw-r----- ahassani/eng     13312  2017-09-28 10:36 disk_ext2_4k_empty.img
-rw-r----- ahassani/eng     20992  2017-09-28 10:37 disk_ext2_unittest.img
-rw-r----- ahassani/eng      4096  2017-09-28 10:37 disk_sqfs_empty.img
-rw-r----- ahassani/eng      4096  2017-09-28 10:37 disk_sqfs_default.img

and here's xvf (toybox only this time, since GNU tar is consistent):

-rw-r----- 1 enh primarygroup 1048064 2017-09-28 10:36 disk_ext2_1k.img
-rw-r----- 1 enh primarygroup 2130432 2017-09-28 10:36 disk_ext2_4k.img
-rw-r----- 1 enh primarygroup  164352 2017-09-28 10:36 disk_ext2_4k_empty.img
-rw-r----- 1 enh primarygroup 2114048 2017-09-28 10:37 disk_ext2_unittest.img
-rw-r----- 1 enh primarygroup    4096 2017-09-28 10:37 disk_sqfs_default.img
-rw-r----- 1 enh primarygroup    4096 2017-09-28 10:37 disk_sqfs_empty.img

md5sum says that the last two files are actually correct, but
(obviously) the first four aren't even the right length. (and
interestingly, toybox tar tvf and toybox tar xvf disagree about the
lengths, even though they're _both_ wrong :-) )

that's all i have for now, but i thought i'd pass it on anyway...