Calculate line size correctly for bit depths < 8

Originally committed as revision 15388 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/bmp.c

@@ -175,7 +175,7 @@ static int bmp_decode_frame(AVCodecContext *avctx,
     dsize = buf_size - hsize;
 
     /* Line size in file multiple of 4 */
-    n = (avctx->width * (depth / 8) + 3) & ~3;
+    n = ((avctx->width * depth) / 8 + 3) & ~3;
 
     if(n * avctx->height > dsize){
         av_log(avctx, AV_LOG_ERROR, "not enough data (%d < %d)\n",