V4L2SVDA: Don't continue with an invalid iterator

BUG=b:112181526
BUG=881669
TEST=build

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Ife2de572cad1a47c0e98a6b9f792cfd962af96f0
Reviewed-on: https://chromium-review.googlesource.com/1212204
Commit-Queue: Pin-chih Lin <johnylin@chromium.org>
Reviewed-by: Pawel Osciak <posciak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589426}

media/gpu/v4l2/v4l2_slice_video_decode_accelerator.cc

@@ -1017,6 +1017,7 @@ void V4L2SliceVideoDecodeAccelerator::Dequeue() {
     if (it == surfaces_at_device_.end()) {
       VLOGF(1) << "Got invalid surface from device.";
       NOTIFY_ERROR(PLATFORM_FAILURE);
+      return;
     }
 
     it->second->SetDecoded();