io_uring: kill extra iovec=NULL in import_iovec()

If io_import_iovec() returns an error, return iovec is undefined and
must not be used, so don't set it to NULL when failing.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>