dix: don't allow for devices with 0 axes

This just makes the existing behavior explicit, previously we relied on
a malloc(numAxes * ...) to return NULL to error out.

dix/devices.c

@@ -1335,6 +1335,7 @@ InitValuatorClassDeviceStruct(DeviceIntPtr dev, int numAxes, Atom *labels,
     ValuatorClassPtr valc;
 
     BUG_RETURN_VAL(dev == NULL, FALSE);
+    BUG_RETURN_VAL(numAxes == 0, FALSE);
 
     if (numAxes > MAX_VALUATORS) {
         LogMessage(X_WARNING,