tx_desc can be dereferenced as a null pointer when count is passed in
as 0.
Signed-off-by: NBruce Allan <bruce.w.allan@intel.com>
Tested-by: NEmil Tantilov <emil.s.tantilov@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>