util: Check for NULL 'dev' on input to usbFreeDevice

Added 'usbFreeDevice' to the useless_free_options list in cfg.mk

cfg.mk

@@ -98,6 +98,7 @@ useless_free_options =				\
   --name=qemuMigrationCookieFree                \
   --name=qemuMigrationCookieGraphicsFree        \
   --name=sexpr_free				\
+  --name=usbFreeDevice                          \
   --name=virBandwidthDefFree			\
   --name=virBitmapFree                          \
   --name=virCPUDefFree				\

src/util/virusb.c

@@ -356,6 +356,8 @@ usbGetDevice(unsigned int bus,
 void
 usbFreeDevice(usbDevice *dev)
 {
+    if (!dev)
+        return;
     VIR_DEBUG("%s %s: freeing", dev->id, dev->name);
     VIR_FREE(dev->path);
     VIR_FREE(dev);
@@ -496,8 +498,7 @@ usbDeviceListDel(usbDeviceList *list,
                  usbDevice *dev)
 {
     usbDevice *ret = usbDeviceListSteal(list, dev);
-    if (ret)
-        usbFreeDevice(ret);
+    usbFreeDevice(ret);
 }
 
 usbDevice *