tar: Handle extra bytes after sparse entries

tar: Handle extra bytes after sparse entries

Skip all entry bytes after sparse entries were encountered. This matches
GNU tar behavior.

I have adjusted (and fixed) the existing test case for this. The test
case test_read_format_gtar_sparse_skip_entry did not work with GNU tar.

In #2558 it was explained that the pax size always overrides the header
size (correct). Since the pax size in the test case was way larger than
the actual entry bytes in archive, GNU tar choke on the test file.

The libarchive parser did not skip any bytes not already read due to
references by sparse entries, so the huge pax size was not detected.

By adjusting the test case to have a leftover byte (only 3 bytes are
referenced through sparse entry now, leaving one extra byte) with a
correct pax size and an invalid header size (after all it is overridden
by pax size), GNU tar works and libarchive gets off its 512 byte
alignment, not being able to read the next entry.

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