avcodec/iff: Factorize av_pix_fmt_desc_get() out

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/iff: check pixfmt for rgb8 / rgbn

Fixes out of array access

Found-by: Piotr Bandurski <ami_stuff@o2.pl>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>