test/nvmf: add null pointer check to subsystem test

Fedora 27 ships with Clang 5 which throws an erroneous error in the
add_ns subsystem test. It believes that subsystem.ns is a null pointer.
In order to circumvent this error, I add an assert to bail out of the
test if the pointer really is null and then only execute the final
assert if the pointer is not null. This way our tests will still pass
Scan-build tests on Fedora27.

Change-Id: I54d3fae485d56033ba5eb23b0aa323480ae6a6e4
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/396051


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>