fio: fix job clone mem leak

fio: fix job clone mem leak

In the loop to create clones at the bottom of add_job the function
get_new_job clones the thread_data, just to occaisonally get the
allocated pointers for filename and files overwritten a few lines later.

The dup files also duplicates the name strings so the references to
these are lost by the setting to null.

This patch fixes takes care of that and frees the memory before
discarding the pointers (found via valgrind).

Signed-off-by: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
Signed-off-by: Jens Axboe <axboe@fb.com>