Add nullptr check.

Bug: 1136327
Change-Id: I8ced300d7de0e2465e54d5111f915af425c66c40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463708
Auto-Submit: David Manouchehri <david@davidmanouchehri.com>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816320}

services/device/usb/usb_device_handle_usbfs.cc

@@ -358,7 +358,7 @@ void UsbDeviceHandleUsbfs::BlockingTaskRunnerHelper::
         FROM_HERE, base::BlockingType::MAY_BLOCK);
     usbdevfs_urb* urb = nullptr;
     int rc = HANDLE_EINTR(ioctl(fd_.get(), USBDEVFS_REAPURBNDELAY, &urb));
-    if (rc) {
+    if (rc || !urb) {
       if (errno == EAGAIN)
         break;
       USB_PLOG(DEBUG) << "Failed to reap urbs";