Prevent crash when color management is turned off

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.

Pull Request: https://projects.blender.org/blender/blender/pulls/111144

source/blender/imbuf/intern/colormanagement.cc

@@ -1355,7 +1355,7 @@ bool IMB_colormanagement_space_is_data(ColorSpace *colorspace)
 
 static void colormanage_ensure_srgb_scene_linear_info(ColorSpace *colorspace)
 {
-  if (!colorspace->info.cached) {
+  if (colorspace && !colorspace->info.cached) {
     OCIO_ConstConfigRcPtr *config = OCIO_getCurrentConfig();
     OCIO_ConstColorSpaceRcPtr *ocio_colorspace = OCIO_configGetColorSpace(config,
                                                                           colorspace->name);