h264: make flush_change() set mmco_reset

This ensures that frames do not get mixed on context reinits

Fixes Ticket2836

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 3c9dd93faa)

libavcodec/h264.c

@@ -2584,6 +2584,7 @@ static void flush_change(H264Context *h)
     h->sync= 0;
     h->list_count = 0;
     h->current_slice = 0;
+    h->mmco_reset = 1;
 }
 
 /* forget old pics after a seek */