fio: fix smalloc strdop allocation failure

fio: fix smalloc strdop allocation failure

smalloc_strdup didn't check for allocation success and thereby ran into
segfaults if the single pool went out of memory.
Now with this patch applied it is still failing, but in a more consistent
way than segfaulting.

You still get a bad allocation, but it looks like this now:
  fio: smalloc OOM
  fio: filesetup.c:1495: dup_files: Assertion `0' failed.
  Aborted

In fact the upper layers expected smalloc_strdup to retrun NULL on failure.

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