Fix --bandwidth-log segmentation fault when numjobs even multiple of 8

Fix --bandwidth-log segmentation fault when numjobs even multiple of 8

Segmentation fault occurs when aggregate bandwidth logging is enabled
(--bandwidth-log) and numjobs is an even multiple of 8. Fault occurs
because logic is using the terminating value of struct thread_data *td
from the most recent for_each_td(). This bug was caught by the
refactoring of for_each_td().

Link: https://github.com/axboe/fio/issues/1534
Signed-off-by: Adam Horshack (horshack@live.com)