mpeg12: Pad framerate tab to 16 entries.

There are many places where we read an unchecked 4-bit index into it.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
(cherry picked from commit dfa37fe8a3)

libavcodec/mpeg12data.c

@@ -305,7 +305,7 @@ const uint8_t ff_mpeg12_mbMotionVectorTable[17][2] = {
 { 0xc, 10 },
 };
 
-const AVRational avpriv_frame_rate_tab[] = {
+const AVRational avpriv_frame_rate_tab[16] = {
     {    0,    0},
     {24000, 1001},
     {   24,    1},