avcodec/h264_refs: Also check reference in ff_h264_build_ref_list()

Fixes: out of array read
Fixes: 15409/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_H264_fuzzer-5758846959616000

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 7d3581e6bb)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_refs.c

@@ -373,9 +373,11 @@ int ff_h264_build_ref_list(H264Context *h, H264SliceContext *sl)
                 av_assert0(0);
             }
 
-            if (i < 0) {
+            if (i < 0 || mismatches_ref(h, ref)) {
                 av_log(h->avctx, AV_LOG_ERROR,
-                       "reference picture missing during reorder\n");
+                       i < 0 ? "reference picture missing during reorder\n" :
+                               "mismatching reference\n"
+                      );
                 memset(&sl->ref_list[list][index], 0, sizeof(sl->ref_list[0][0])); // FIXME
             } else {
                 for (i = index; i + 1 < sl->ref_count[list]; i++) {