nodedev: handle failure to generate mdevctl cmd

Coverity complained that the 'default' case of the switch in
nodeDeviceGetMdevctlCommand() was falling through without initializing
'cmd'. Return NULL in this case even though it should never happen.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

src/node_device/node_device_driver.c

@@ -743,7 +743,7 @@ nodeDeviceGetMdevctlCommand(virNodeDeviceDef *def,
     case MDEVCTL_CMD_LAST:
     default:
         /* SHOULD NEVER HAPPEN */
-        break;
+        return NULL;
     }
 
     switch (cmd_type) {