ports: Don't crash with an assertion error when no hook can be found

ports: Don't crash with an assertion error when no hook can be found

This happens when a device has an empty configuration file or
the HOOK variable is not set for an other reason.

In that case the function make the entire program crash.

This patch changes behaviour in that sense that the function
returns with an error which must be evaluated by the calling
function.

Fixes #11400

Reported-by: Daniel Weismüller <daniel.weismueller@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>