yop: initialize palette to 0

The FATE sample contains some pixels with value 0, but the palette
stored in the file contains only values from 16 up. Because the default
and cmdutils get_buffer() initialize the data to 0x80, they appear as
gray dots.
After this commit they change to black dots, which is probably still
incorrect but less visible and doesn't rely on get_buffer() initializing
the data.

libavcodec/yop.c

@@ -206,6 +206,9 @@ static int yop_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
         return ret;
     }
 
+    if (!avctx->frame_number)
+        memset(s->frame.data[1], 0, AVPALETTE_SIZE);
+
     s->dstbuf     = s->frame.data[0];
     s->dstptr     = s->frame.data[0];
     s->srcptr     = avpkt->data + 4;

tests/ref/fate/yop

 #tb 0: 1/12
 0,          0,          0,        1,   302760, 0x78939253
 0,          1,          1,        1,   302760, 0x534f5253
-0,          2,          2,        1,   302760, 0xe991aa82
-0,          3,          3,        1,   302760, 0xc34b20bd
-0,          4,          4,        1,   302760, 0x461d29a1
+0,          2,          2,        1,   302760, 0x25eaa782
+0,          3,          3,        1,   302760, 0x60861c3d
+0,          4,          4,        1,   302760, 0x43552521
 0,          5,          5,        1,   302760, 0x45abca02