Fix: don't hide EBADF error subbuffer write

Fix: don't hide EBADF error subbuffer write

There are _no_ legitimate circumstances where a "EBADF" error should
be considered normal in a multi-threaded program.

This doesn't fix a known problem, but it hid an error while testing a
development branch. It is considered a fix since not logging this
error can make debugging more difficult if this was, in fact, hiding a
problem.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>