net/failsafe: fix blank line parsing

net/failsafe: fix blank line parsing

When the output of an exec() slave definition is only a single newline
character, the fail-safe currently fails to parse the device with the
value returned by the rte_devargs library.

This behavior is incorrect, because the fail-safe should make a
difference between the absence of a device, and an erroneous device
declaration.

Fix the output sanitization in the case where no newline was at its end
and detect the special case of an absent device. The correct error code
is then returned.

Fixes: a0194d828100 ("net/failsafe: add flexible device definition")

Signed-off-by: Gaetan Rivet <gaetan.rivet@6wind.com>