Graciously return non-variable advance when glyf-based advance fails

[drott]

In get_advance_with_var_unscaled when the font is variable, first an HVAR based advance is attempted to be computed. If that fails, a glyf based / phantom-points advance is to be computed.

This value is unconditionally returned, even though the glyf based advance with computation could fail (for example when there is no glyf table).

IMO in this case, it'd be more robust behavior to still return the non-variable advance from line 402:
unsigned int advance = get_advance_without_var_unscaled (glyph);

Can we do sth like:

     unsigned int glyf_advance = _glyf_get_advance_with_var_unscaled (font, glyph, T::tableTag == HB_OT_TAG_vmtx);
      if (!glyf_advance) {
        return advance;
      } else {
        return glyf_advance;
      }

[drott]

CC @nedley

[drott]

Thanks, @behdad !