hostname: fix panic on hostname > /dev/full

[WhateverAWS]

fixes #10812

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/pr/bounded-memory. tests/pr/bounded-memory is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/tail/follow-name (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)
Congrats! The gnu test tests/printf/printf-surprise is now passing!

[ChrisDryden]

Just making a note here for future reference that there should be no need for doing a single lock of the stdout since only one thing should be printing.

Thanks!