avcodec/vc1dec: zero SpriteData struct

Fixes use of uninitialized data, as alternative alpha could be
calculated conditionally

Fixes part of msan_uninit-mem_7f51a8b0b3b0_1009_Arlington.wmv
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vc1dec.c

@@ -5459,6 +5459,8 @@ static int vc1_decode_sprites(VC1Context *v, GetBitContext* gb)
     AVCodecContext *avctx = s->avctx;
     SpriteData sd;
 
+    memset(&sd, 0, sizeof(sd));
+
     vc1_parse_sprites(v, gb, &sd);
 
     if (!s->current_picture.f.data[0]) {