tree: Handle NULL subsysname in nvme_scan_ctrl()

[tbzatek]

As noted couple of lines earlier, the subsysname string might
be NULL. In that case, defer initialization of nvme_subsystem_t
in hope that it would get initialized while scanning subsystems.

[tbzatek]

This is a followup to #304 - Github was somewhat broken yesterday, giving me Internal server error on any git push and eventually succeeded... only to make my changes ignored in the webui and as checked after the merge, also in the merged code.

Resubmitting, just a tiny fix.

[igaw]

Thanks!