support: Also return fd when it is 0

The fd validity check in open_dev_null checks if fd > 0, which would
lead to a leaked fd if it is == 0.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
(cherry picked from commit 27b6edbb)