bmp: fix decoding of flipped rle4

Fixes ticket #2794
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/bmp.c

@@ -256,7 +256,7 @@ static int bmp_decode_frame(AVCodecContext *avctx,
         buf = buf0 + hsize;
     }
     if (comp == BMP_RLE4 || comp == BMP_RLE8) {
-        if (height < 0) {
+        if (comp == BMP_RLE8 && height < 0) {
             p->data[0]    +=  p->linesize[0] * (avctx->height - 1);
             p->linesize[0] = -p->linesize[0];
         }