lavc/rawdec: add assertion check in raw_decode()

Check on an implicit assumption done on the number of coded bits for
paletted rawvideo with coded bits == 2.

libavcodec/rawdec.c

@@ -27,6 +27,7 @@
 #include "avcodec.h"
 #include "imgconvert.h"
 #include "raw.h"
+#include "libavutil/avassert.h"
 #include "libavutil/intreadwrite.h"
 #include "libavutil/imgutils.h"
 #include "libavutil/opt.h"
@@ -179,6 +180,7 @@ static int raw_decode(AVCodecContext *avctx,
             }
             linesize_align = 8;
         } else {
+            av_assert0(avctx->bits_per_coded_sample == 2);
             for(i=0; 4*i+3 < buf_size && i<avpkt->size; i++){
                 dst[4*i+0]= buf[i]>>6;
                 dst[4*i+1]= buf[i]>>4&3;