decoder_get_frame() should use output frame index.

decoder_get_frame() should use pbi->output_frame_index[*index], not
cm->new_fb_idx. After decoding an output frame, if the remaining input
data are valid OBUs but don't contain a frame, cm->new_fb_idx will be
released and not copied to the pbi->output_frame_index[] array.

Tested:
  ./test_libaom --gtest_filter=*InvalidFileTest*
  ./test_libaom --gtest_filter=*ExternalFrameBuffer*
  ./test_libaom --gtest_filter=*TestVectorTest*

BUG=chromium:906381

Change-Id: Id20f8d25fe8809445d917c6822a0edd9a132483a
diff --git a/test/test_data_util.cmake b/test/test_data_util.cmake
index 629b7bf..c029fad 100644
--- a/test/test_data_util.cmake
+++ b/test/test_data_util.cmake
@@ -508,6 +508,8 @@
               "av1-1-b8-22-svc-L2T2.ivf.md5"
               "invalid-bug-1814.ivf"
               "invalid-bug-1814.ivf.res"
+              "invalid-chromium-906381.ivf"
+              "invalid-chromium-906381.ivf.res"
               "invalid-oss-fuzz-10061.ivf"
               "invalid-oss-fuzz-10061.ivf.res"
               "invalid-oss-fuzz-10117-mc-buf-use-highbd.ivf"