nvme_disconnect_ctrl followed by a lookup will crash

[igaw]

As discovered in linux-nvme/nvme-cli#2674 something is amiss when a controller is disconnected and then a lookup happens (tree iteration), this will trigger a crash in strcmp

nvme_disconnect_ctrl() 
    nvme_deconfigure_ctrl() 
lookup_nvme_ctrl() 
   strcmp(nvme_ctrl_get_name(c), name)