linux: avoid segfault in check-tls-key due to null hostnqn/subsysnqn

linux: avoid segfault in check-tls-key due to null hostnqn/subsysnqn

Running nvme check-tls-key hits a segfault as seen below:

nvme check-tls-key
-d NVMeTLSkey-1:01:bB7soUnpHfxVg53sCY21KY3nLbqLit2RcIO8Rbdf3mKhcKaM:

Segmentation fault (core dumped)

This is because the strlen check on subsysnqn or hostnqn crashes at
src/nvme/linux.c due to either of them being null. Avoid this segfault
by checking these strings before running a strlen on them.

Signed-off-by: Martin George <marting@netapp.com>