netdev_get_qos returns a value to indicate if an error has occurred while
attempting to query the QoS configuration of an interface. If an error does
occur the pointer argument passed to it will be set to null before returning.
Currently the vswitch will segfault if this occurs as qos_unixctl_show will
attempt to access the pointer directly after it calls netdev_get_qos.

Avoid this by adding a check for the return value and flagging an appropriate
error message to appctl.

Signed-off-by: Ian Stokes <ian.stokes@intel.com>
[blp@ovn.org changed details of error report]
Signed-off-by: Ben Pfaff <blp@ovn.org>