dd: catch OOM

[oech3]

• Closes dd bs=10000000G SIGABRT #11436
• Declare buffer before we use it (preliminary to avoid 0-filling)

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/io-errors. tests/misc/io-errors is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/rm/isatty (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/csplit/csplit-heap is now passing!
Congrats! The gnu test tests/tail/tail-n0f is now passing!
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/rm/isatty (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cut/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/csplit/csplit-heap is now passing!
Congrats! The gnu test tests/tail/tail-n0f is now passing!
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.