Prevent a segmentation fault when the viewport is set to 'Material Preview'
mode, drawing textures, and Blender built with `WITH_OPENCOLORIO=OFF`.
Most `IMB_colormanagement_...` functions assume that the `colorspace`
pointer can be `nullptr`, but the internal function
`colormanage_ensure_srgb_scene_linear_info()` does not, causing a crash.
Since this check can be done in multiple places, I picked one that seems
consistent with the rest of the code.
Sybren A. Stüvel