tar: Handle many sparse comments on 32 bit systems

tar: Handle many sparse comments on 32 bit systems

The sparse 1.0 parser skips lines with comments. The amount of skipped
bytes is stored in a ssize_t variable, although common 32 bit systems
allow files larger than 4 GB.

Gracefully handle files with more than 2 GB bytes full of comments to
prevent integer truncations.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>