Correctly decode some Avid AV1x samples, fixes issue 1684.

Originally committed as revision 21241 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/rawdec.c

@@ -137,6 +137,9 @@ static int raw_decode(AVCodecContext *avctx,
         buf= dst;
     }
 
+    if(avctx->codec_tag == MKTAG('A', 'V', '1', 'x'))
+        buf += buf_size - context->length;
+
     if(buf_size < context->length - (avctx->pix_fmt==PIX_FMT_PAL8 ? 256*4 : 0))
         return -1;