mpegvideo: increase scratchpad sizes

These where reduced in f1d8763a
and this change can cause out of array accesses.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpegvideo.c

@@ -244,7 +244,7 @@ int ff_mpv_frame_size_alloc(MpegEncContext *s, int linesize)
     FF_ALLOCZ_OR_GOTO(s->avctx, s->edge_emu_buffer, alloc_size * 4 * 24,
                       fail);
 
-    FF_ALLOCZ_OR_GOTO(s->avctx, s->me.scratchpad, alloc_size * 2 * 16 * 2,
+    FF_ALLOCZ_OR_GOTO(s->avctx, s->me.scratchpad, alloc_size * 4 * 16 * 2,
                       fail)
     s->me.temp         = s->me.scratchpad;
     s->rd_scratchpad   = s->me.scratchpad;