OBU_FRAME type requires show_existing_frame == 0.

Add a check for the following requirement in Section 6.8.2 of the spec:

  6.8.2. Uncompressed header semantics

  If obu_type is equal to OBU_FRAME, it is a requirement of
  bitstream conformance that show_existing_frame is equal to 0.

BUG=aomedia:2030

Change-Id: I3ae68d65589ff605be88ff4c5eb02d1cf9d52ccc

av1/decoder/obu.c

diff --git a/av1/decoder/obu.c b/av1/decoder/obu.c
index 37de404..897a41e 100644
--- a/av1/decoder/obu.c
+++ b/av1/decoder/obu.c
@@ -836,6 +836,10 @@
         pbi->frame_header_size = (size_t)frame_header_size;
 
         if (cm->show_existing_frame) {
+          if (obu_header.type == OBU_FRAME) {
+            cm->error.error_code = AOM_CODEC_UNSUP_BITSTREAM;
+            return -1;
+          }
           frame_decoding_finished = 1;
           pbi->seen_frame_header = 0;
           break;