Use AVPALETTE_COUNT instead of hard-coding 256.

This is consistent with other codecs and will also avoid a crash on the
memcpy to data[1] if AVPALETTE_SIZE ever increases.

Originally committed as revision 21399 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/vb.c

@@ -43,7 +43,7 @@ typedef struct VBDecContext {
     AVFrame pic;
 
     uint8_t *frame, *prev_frame;
-    uint32_t pal[256];
+    uint32_t pal[AVPALETTE_COUNT];
     const uint8_t *stream;
 } VBDecContext;