avcodec/h264dec: Fix potential array overread

add padding before scantable arrays

See: 522d850eSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 380b48fb)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264.h

@@ -521,6 +521,7 @@ typedef struct H264Context {
     uint8_t *direct_table;
     uint8_t direct_cache[5 * 8];
 
+    uint8_t scan_padding[16];
     uint8_t zigzag_scan[16];
     uint8_t zigzag_scan8x8[64];
     uint8_t zigzag_scan8x8_cavlc[64];