dd: avoid 0-filling buf at read_and_discard

[oech3]

$ truncate -s 64GB huge
$ cat huge |target/release/dd-avoid of=/dev/null bs=128M skip=64GB
0+0 records in
0+0 records out
0 bytes copied, 0.0173446 s, 0.0 B/s
$ cat huge |target/release/dd of=/dev/null bs=128M skip=64GB
0+0 records in
0+0 records out
0 bytes copied, 0.0178161 s, 0.0 B/s

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/unexpand/bounded-memory is now being skipped but was previously passing.

[codspeed-hq]

Merging this PR will not alter performance

✅ 303 untouched benchmarks
⏩ 46 skipped benchmarks¹

---

_{Comparing oech3:dd-bufreader (4c21774) with main (1e81775)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[oech3]

I have no idea why do I see serious perf drop with fn fill_consecutive() by avoiding 0-filling.