mpegvideo: allocate sufficiently large scratch buffer for interlaced vid

MPV_decode_mb_internal needs 3 * 16 * linesize bytes of scratch buffer

For interlaced content, linesize is multiplied by two after the allocation
of the scratch buffer, and the dest_cr pointer ends past the buffer.

This patch makes ff_mpv_frame_size_alloc allocate a total of
(aligned line_size) * 2 * 16 * 3 bytes, which suffices even for the
interlaced case.

CC:libav-stable@libav.org
Signed-off-by: Jindrich Makovicka <makovick@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/mpegvideo.c

@@ -205,7 +205,7 @@ int ff_mpv_frame_size_alloc(MpegEncContext *s, int linesize)
     FF_ALLOCZ_OR_GOTO(s->avctx, s->edge_emu_buffer, alloc_size * 2 * 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 * 2 * 16 * 3,
                       fail)
     s->me.temp         = s->me.scratchpad;
     s->rd_scratchpad   = s->me.scratchpad;