[Backport] CVE-2021-21140: Uninitialized Use in USB [1/2]

Cherry-pick of patch originally reviewed on
https://chromium-review.googlesource.com/c/chromium/src/+/2459447:
Initialize urb to make MSan happy.

Bug: 1136327
Change-Id: Id47e06bfd088e5c72d5f41c091882e99768fb93d
Commit-Queue: Reilly Grant <[email protected]>
Auto-Submit: David Manouchehri <[email protected]>
Reviewed-by: Reilly Grant <[email protected]>
Cr-Commit-Position: refs/heads/master@{#815025}
Reviewed-by: Allan Sandfeld Jensen <[email protected]>
(cherry picked from commit f195542)

Author: Manouchehri

chromium/device/usb/usb_device_handle_usbfs.cc

@@ -328,7 +328,7 @@ void UsbDeviceHandleUsbfs::FileThreadHelper::OnFileCanWriteWithoutBlocking() {
   std::vector<usbdevfs_urb*> urbs;
   urbs.reserve(MAX_URBS_PER_EVENT);
   for (size_t i = 0; i < MAX_URBS_PER_EVENT; ++i) {
-    usbdevfs_urb* urb;
+    usbdevfs_urb* urb = nullptr;
     int rc = HANDLE_EINTR(ioctl(fd_.get(), USBDEVFS_REAPURBNDELAY, &urb));
     if (rc) {
       if (errno == EAGAIN)