Ensure kerning cache exists when loading embedded fonts

When loading embedded fonts from memory the font->kerning_cache is not created and so we get a crash if the font does support kerning. This was not caught because the loading of packed fonts was not actually doing anything in VSE until {523bc981cfee}.

We have since consolidated blf_font_new and blf_font_new_from_mem into a single function so that they cannot get out of sync like this any more. So this fix is specific to Blender 3.3. But we can add this as a candidate for corrective release 3.2.3