avcodec/pthread_slice: Use av_freep() to avoid leaving stale pointers in memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/pthread_slice.c

@@ -120,7 +120,7 @@ void ff_slice_thread_free(AVCodecContext *avctx)
     pthread_mutex_destroy(&c->current_job_lock);
     pthread_cond_destroy(&c->current_job_cond);
     pthread_cond_destroy(&c->last_job_cond);
-    av_free(c->workers);
+    av_freep(&c->workers);
     av_freep(&avctx->internal->thread_ctx);
 }