Flush avcodec on discontinuity.

It is needed to be sure that the first image displayed after a discontinuity
(flush or seek) is the right one.

Manually cherry-picked from 4979e6d702eab1973dfee025c492bb3f945363e0 by jb
Please, double check

modules/codec/avcodec/video.c

@@ -430,10 +430,10 @@ picture_t *DecodeVideo( decoder_t *p_dec, block_t **pp_block )
         p_sys->input_pts = p_sys->input_dts = 0;
         p_sys->i_late_frames = 0;
 
-        block_Release( p_block );
+        if( p_block->i_flags & BLOCK_FLAG_DISCONTINUITY )
+            avcodec_flush_buffers( p_sys->p_context );
 
-        //if( p_block->i_flags & BLOCK_FLAG_CORRUPTED )
-            //avcodec_flush_buffers( p_sys->p_context );
+        block_Release( p_block );
         return NULL;
     }