mpegaudiodec: Enable checked bitstream reader.

It appears there are corner cases with damaged input that can lead
to small overreads.

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

libavcodec/mpegaudiodec.c

@@ -24,8 +24,6 @@
  * MPEG Audio decoder
  */
 
-#define UNCHECKED_BITSTREAM_READER 1
-
 #include "libavutil/audioconvert.h"
 #include "avcodec.h"
 #include "get_bits.h"