wmaker: fix signedness of variable (Coverity #50082, #50222)
Coverity complain that there can be security issues because the variable
'i' is being modified using untrusted data (coming from a file). This is
probably pessimistic, because in the present case we're talking with the
kernel.
Using the correct signedness for the variable should however keep us safe,
and (I hope) make Coverity happy.
Took opportunity to include an error message in case of read problem
because it can help to debug.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>