avcodec/hevcdec: Check ref frame

Fixes: NULL pointer dereferences
Fixes: 68197/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-6382538823106560

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

libavcodec/hevcdec.c

@@ -1969,13 +1969,13 @@ static void hls_prediction_unit(HEVCLocalContext *lc, int x0, int y0,
 
     if (current_mv.pred_flag & PF_L0) {
         ref0 = refPicList[0].ref[current_mv.ref_idx[0]];
-        if (!ref0 || !ref0->frame->data[0])
+        if (!ref0 || !ref0->frame)
             return;
         hevc_await_progress(s, ref0, &current_mv.mv[0], y0, nPbH);
     }
     if (current_mv.pred_flag & PF_L1) {
         ref1 = refPicList[1].ref[current_mv.ref_idx[1]];
-        if (!ref1 || !ref1->frame->data[0])
+        if (!ref1 || !ref1->frame)
             return;
         hevc_await_progress(s, ref1, &current_mv.mv[1], y0, nPbH);
     }