avcodec/hevc.c: for big negative mvy value, should wait line 0 of ref frame due to edge extending

Reviewed-by: Mickaël Raulet <mraulet@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/hevc.c

@@ -1571,7 +1571,7 @@ static void chroma_mc_bi(HEVCContext *s, uint8_t *dst0, ptrdiff_t dststride, AVF
 static void hevc_await_progress(HEVCContext *s, HEVCFrame *ref,
                                 const Mv *mv, int y0, int height)
 {
-    int y = (mv->y >> 2) + y0 + height + 9;
+    int y = FFMAX(0, (mv->y >> 2) + y0 + height + 9);
 
     if (s->threads_type == FF_THREAD_FRAME )
         ff_thread_await_progress(&ref->tf, y, 0);