jfs: add a check to prevent array-index-out-of-bounds in dbAdjTree

When the value of lp is 0 at the beginning of the for loop, it will
become negative in the next assignment and we should bail out.

Reported-by:  <syzbot+412dea214d8baa3f7483@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=412dea214d8baa3f7483


Tested-by:  <syzbot+412dea214d8baa3f7483@syzkaller.appspotmail.com>
Signed-off-by: Nihar Chaithanya <niharchaithanya@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>