Fix occasional NaNs in alpha channel with 3 channel images.
Fix occasional NaNs in alpha channel with 3 channel images.

YangYangTL/oiio,lgritz/oiio,OpenImageIO/oiio,YangYangTL/oiio,lgritz/oiio,micler/oiio,OpenImageIO/oiio,scott-wilson/oiio,scott-wilson/oiio,lgritz/oiio,micler/oiio,bdeluca/oiio,cwilling/oiio,mcanthony/oiio,OpenImageIO/oiio,lgritz/oiio,cwilling/oiio,OpenImageIO/oiio,mcanthony/oiio,micler/oiio,mcanthony/oiio,mcanthony/oiio,bdeluca/oiio,YangYangTL/oiio,scott-wilson/oiio,sambler/oiio,cwilling/oiio,YangYangTL/oiio,bdeluca/oiio,micler/oiio,sambler/oiio,sambler/oiio,cwilling/oiio,sambler/oiio,bdeluca/oiio,scott-wilson/oiio