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>

src/jobs.c

@@ -699,7 +699,7 @@ check:
 	if (is_number(p)) {
 		num = atoi(p);
-		if (num <= njobs) {
+		if (num > 0 && num <= njobs) {
 			jp = jobtab + num - 1;
 			if (jp->used)
 				goto gotit;