avcodec/h264_slice: Clear ref_counts on redundant slices

Fixes reading freed memory
Fixes: 568/clusterfuzz-testcase-6107186067406848

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit c03029a835)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264.c

@@ -1023,7 +1023,8 @@ again:
 #endif
                 } else
                     context_count++;
-            }
+            } else
+                sl->ref_count[0] = sl->ref_count[1] = 0;
             break;
         case NAL_DPA:
         case NAL_DPB: