Fix syntax sync issue for show_existing_frame = 1

Prior to this patch, a zero byte was written to the bitstream in
write_uncompressed_header_obu() when cm->show_existing_frame == 1
and cm->seq_params.frame_id_numbers_present_flag == 1. However,
this byte is not read in read_uncompressed_header(), resulting in
a mismatch in header sizes.

This patch resolves this issue by reading and discarding the zero
byte in read_uncompressed_header().

BUG=aomedia:1579

Change-Id: I78f722aa46ca4a5c3a6a571e9e17baa0ea1a1dcb

av1/decoder/decodeframe.c

diff --git a/av1/decoder/decodeframe.c b/av1/decoder/decodeframe.c
index 59de549..c783636 100644
--- a/av1/decoder/decodeframe.c
+++ b/av1/decoder/decodeframe.c
@@ -2656,6 +2656,8 @@
     if (cm->seq_params.frame_id_numbers_present_flag) {
       int frame_id_length = cm->seq_params.frame_id_length;
       int display_frame_id = aom_rb_read_literal(rb, frame_id_length);
+      // Read and discard one byte
+      aom_rb_read_literal(rb, 8);
       /* Compare display_frame_id with ref_frame_id and check valid for
        * referencing */
       if (display_frame_id != cm->ref_frame_id[existing_frame_idx] ||