[HarfBuzz] How to make sure an hb_font_t object is valid?

[Eli Zaretskii]

Last time I asked a similar question, I was told to use
hb_face_get_glyph_count.  But eventually I need to know that an
hb_font_t I create from the face is valid and can be used for
shaping.  What are the best practices for doing that?

Or maybe the shaper will return 'false' when given an invalid font,
and all I need is to test the return value of the shaper?

TIA