avcodec/dts2pts_bsf: Check ctx for NULL before ff_cbs_flush()

Fixes: null pointer dereference
Fixes: 52155/clusterfuzz-testcase-minimized-ffmpeg_BSF_DTS2PTS_fuzzer-5760107527143424

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/dts2pts_bsf.c

@@ -505,6 +505,7 @@ static void dts2pts_flush(AVBSFContext *ctx)
     s->root = NULL;
 
     ff_cbs_fragment_reset(&s->au);
+    if (s->cbc)
         ff_cbs_flush(s->cbc);
 }