avcodec/hevcdec: Avoid only partly skiping duplicate first slices

Fixes: NULL pointer dereference and out of array access
Fixes: 13871/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-5746167087890432
Fixes: 13845/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-5650370728034304

This also fixes the return code for explode mode

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>
(cherry picked from commit 54655623a8)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/hevc.c

@@ -439,6 +439,11 @@ static int hls_slice_header(HEVCContext *s)
 
     // Coded parameters
     sh->first_slice_in_pic_flag = get_bits1(gb);
+    if (s->ref && sh->first_slice_in_pic_flag) {
+        av_log(s->avctx, AV_LOG_ERROR, "Two slices reporting being the first in the same frame.\n");
+        return 1; // This slice will be skiped later, do not corrupt state
+    }
+
     if ((IS_IDR(s) || IS_BLA(s)) && sh->first_slice_in_pic_flag) {
         s->seq_decode = (s->seq_decode + 1) & 0xff;
         s->max_ra     = INT_MAX;
@@ -2775,6 +2780,11 @@ static int decode_nal_unit(HEVCContext *s, const H2645NAL *nal)
         ret = hls_slice_header(s);
         if (ret < 0)
             return ret;
+        if (ret == 1) {
+            ret = AVERROR_INVALIDDATA;
+            goto fail;
+        }
+
 
         if (s->max_ra == INT_MAX) {
             if (s->nal_unit_type == NAL_CRA_NUT || IS_BLA(s)) {
@@ -2795,10 +2805,6 @@ static int decode_nal_unit(HEVCContext *s, const H2645NAL *nal)
         }
 
         if (s->sh.first_slice_in_pic_flag) {
-            if (s->ref) {
-                av_log(s->avctx, AV_LOG_ERROR, "Two slices reporting being the first in the same frame.\n");
-                goto fail;
-            }
             ret = hevc_frame_start(s);
             if (ret < 0)
                 return ret;