ffplay: fix use after free

reproduceable with:
ffmpeg -i tests/lena.pnm -pix_fmt pal8 -vcodec rawvideo -s 512x512 out.avi
valgrind ffplay_g out.avi

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Marton Balint <cus@passwd.hu>
