Correctly set seen_frame_header

Set seen_frame_header after read_frame_header_obu() completes
successfully. It should not be set if read_frame_header_obu() triggers
internal error.

BUG=oss-fuzz:8855
BUG=b/110329580

Change-Id: Ie678bc3294e1e269ef5443a5b36b4db6042029ed
diff --git a/av1/decoder/obu.c b/av1/decoder/obu.c
index a6b5131..fdcaec0 100644
--- a/av1/decoder/obu.c
+++ b/av1/decoder/obu.c
@@ -812,9 +812,9 @@
         // Only decode first frame header received
         if (!pbi->seen_frame_header ||
             (cm->large_scale_tile && !pbi->camera_frame_header_ready)) {
-          pbi->seen_frame_header = 1;
           frame_header_size = read_frame_header_obu(
               pbi, &rb, data, p_data_end, obu_header.type != OBU_FRAME);
+          pbi->seen_frame_header = 1;
           if (!pbi->ext_tile_debug && cm->large_scale_tile)
             pbi->camera_frame_header_ready = 1;
         }