avcodec/pthread_frame: Fix leak of coded side data

Happens in the mov-elst-ends-betn-b-and-i and mov-ibi-elst-starts-b
FATE tests with frame-threading.

Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/pthread_frame.c

@@ -731,6 +731,8 @@ void ff_frame_thread_free(AVCodecContext *avctx, int thread_count)
             av_packet_free(&ctx->internal->last_pkt_props);
             av_freep(&ctx->internal);
             av_buffer_unref(&ctx->hw_frames_ctx);
+            av_frame_side_data_free(&ctx->decoded_side_data,
+                                    &ctx->nb_decoded_side_data);
         }
 
         av_frame_free(&p->frame);