ash: jobs: Don't attempt to access job table for job %0

ash: jobs: Don't attempt to access job table for job %0

Upstream commit:

    jobs: Don't attempt to access job table for job %0

    If job %0 is (mistakenly) specified, an out-of-bounds access to the
    jobtab occurs in function getjob() if num = 0:

            jp = jobtab + 0 - 1

    Fix this by checking that the job number is larger than 0 before
    accessing the jobtab.

    Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>