avformat/avidec: Check for dv streams before using priv_data in parse ##dc/##wb

Fixes: null pointer dereference
Fixes: 31588/clusterfuzz-testcase-minimized-ffmpeg_dem_AVI_fuzzer-6165716135968768

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

libavformat/avidec.c

@@ -1288,7 +1288,7 @@ start_sync:
                 AVStream *st1   = s->streams[1];
                 AVIStream *ast1 = st1->priv_data;
                 // workaround for broken small-file-bug402.avi
-                if (   d[2] == 'w' && d[3] == 'b'
+                if (ast1 && d[2] == 'w' && d[3] == 'b'
                    && n == 0
                    && st ->codecpar->codec_type == AVMEDIA_TYPE_VIDEO
                    && st1->codecpar->codec_type == AVMEDIA_TYPE_AUDIO