mpegvideo: fix size of array

CC: libav-stable@libav.org
Bug-Id: CID 1238789

libavcodec/mpegvideo.h

@@ -600,7 +600,7 @@ typedef struct MpegEncContext {
     int16_t (*pblocks[12])[64];
 
     int16_t (*block)[64]; ///< points to one of the following blocks
-    int16_t (*blocks)[8][64]; // for HQ mode we need to keep the best block
+    int16_t (*blocks)[12][64]; // for HQ mode we need to keep the best block
     int (*decode_mb)(struct MpegEncContext *s, int16_t block[6][64]); // used by some codecs to avoid a switch()
 #define SLICE_OK         0
 #define SLICE_ERROR     -1