Fix linesize for bmp with bitdepth < 8.

Fixes ticket #751.

libavcodec/bmp.c

@@ -206,7 +206,7 @@ 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 + 31) / 8) & ~3;
 
     if(n * avctx->height > dsize && comp != BMP_RLE4 && comp != BMP_RLE8){
         av_log(avctx, AV_LOG_ERROR, "not enough data (%d < %d)\n",