io_uring: only call kfree() for a non-zero pointer

It's safe to call kfree() with a NULL pointer, but it's also pointless.
Most of the time we don't have any data to free, and at millions of
requests per second, the redundant function call adds noticeable
overhead (about 1.3% of the runtime).

Signed-off-by: Jens Axboe <axboe@kernel.dk>