Fix crash on thread exit

If we had multiple threads, we could corrupt the heap by
freeing memory we did not alloc.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/ioengines.c b/ioengines.c
index 3943320..441f36f 100644
--- a/ioengines.c
+++ b/ioengines.c
@@ -164,7 +164,10 @@
 	if (td->io_ops->dlhandle)
 		dlclose(td->io_ops->dlhandle);
 
+#if 0
+	/* we can't do this for threads, so just leak it, it's exiting */
 	free(td->io_ops);
+#endif
 	td->io_ops = NULL;
 }