avutil/frame: suppress "comparison of unsigned expression < 0 is always false" warning

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swscale/swscale_unscaled: fix alpha values for rgb/bgr -> RGB32_1 / BGR32_1

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