ice: Check for q_vector when stopping rings

There is a gap in time between a VF reset, which sets the q_vector to
NULL, and the VF requesting mapping of the q_vectors. If
ice_vsi_stop_tx_rings() is called during this time, a NULL pointer
dereference is encountered. Add a check in ice_vsi_stop_tx_rings()
to ensure the q_vector is set to avoid this situation from occurring.

Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>