avcodec: put back avcodec_flush_buffers on pf_flush

Fixes #15834

modules/codec/avcodec/video.c

@@ -502,14 +502,14 @@ int InitVideoDec( decoder_t *p_dec, AVCodecContext *p_context,
 static void Flush( decoder_t *p_dec )
 {
     decoder_sys_t *p_sys = p_dec->p_sys;
+    AVCodecContext *p_context = p_sys->p_context;
 
     p_sys->i_pts = VLC_TS_INVALID; /* To make sure we recover properly */
     p_sys->i_late_frames = 0;
-#if 0
+
     post_mt( p_sys );
     avcodec_flush_buffers( p_context );
     wait_mt( p_sys );
-#endif
 }
 
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