Check return value of scandir

Check return value of scandir

On error, scandir returns -1 and does not allocate memory
for namelist array. In some places in the code return value
of scandir call is not checked. This causes nvme-cli to
attempt to free() an uninitialized pointer, which subsequently
leads to segmentation fault.

To address this issue, check return value of scandir calls
throughout the code.

Signed-off-by: Szczerbik, Przemyslaw <przemyslawx.szczerbik@intel.com>