cmd: Add test and fix bugs for dm drivers

Add a test for the dm drivers command. Also fix a null pointer dereference
revealed by said test.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>