avcodec/av1dec: Check for unset obu instead of crashing

Fixes: NULL pointer dereference
Fixes: 26550/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AV1_fuzzer-5417762807349248

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/av1dec.c

@@ -673,7 +673,12 @@ static int av1_decode_frame(AVCodecContext *avctx, void *frame,
     for (int i = 0; i < s->current_obu.nb_units; i++) {
         CodedBitstreamUnit *unit = &s->current_obu.units[i];
         AV1RawOBU *obu = unit->content;
-        const AV1RawOBUHeader *header = &obu->header;
+        const AV1RawOBUHeader *header;
+
+        if (!obu)
+            continue;
+
+        header = &obu->header;
         av_log(avctx, AV_LOG_DEBUG, "Obu idx:%d, obu type:%d.\n", i, unit->type);
 
         switch (unit->type) {