swscale/yuv2rgb: Fix width % 4 != 0 with bgr4/rgb4

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swscale/yuv2rgb: Fix width % 4 != 0 with bgr4_byte/rgb4_byte

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>