mpeg12data: increase size of ff_mpeg1_default_intra_matrix to prevent harmless overreads from crashing

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpeg12data.c

@@ -27,7 +27,7 @@
 
 #include "mpeg12data.h"
 
-const uint16_t ff_mpeg1_default_intra_matrix[64] = {
+const uint16_t ff_mpeg1_default_intra_matrix[256] = {
         8, 16, 19, 22, 26, 27, 29, 34,
         16, 16, 22, 24, 27, 29, 34, 37,
         19, 22, 26, 27, 29, 34, 34, 38,

libavcodec/mpeg12data.h

@@ -32,7 +32,7 @@
 #include "libavutil/rational.h"
 #include "rl.h"
 
-extern const uint16_t ff_mpeg1_default_intra_matrix[64];
+extern const uint16_t ff_mpeg1_default_intra_matrix[];
 extern const uint16_t ff_mpeg1_default_non_intra_matrix[64];
 
 extern const uint16_t ff_mpeg12_vlc_dc_lum_code[12];