e1000e: prevent null ptr dereference in e1000_tx_queue()

tx_desc can be dereferenced as a null pointer when count is passed in
as 0.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Emil Tantilov <emil.s.tantilov@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>