avcodec: increase padding, fixes ticket313

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/avcodec.h

@@ -429,7 +429,7 @@ enum CodecID {
  * Note: If the first 23 bits of the additional bytes are not 0, then damaged
  * MPEG bitstreams could cause overread and segfault.
  */
-#define FF_INPUT_BUFFER_PADDING_SIZE 8
+#define FF_INPUT_BUFFER_PADDING_SIZE 16
 
 /**
  * minimum encoding buffer size