Fix state of `FT_Face' for buggy `gvar' tables (#57923).

By resetting the blend as implemented with this commit fonts with
invalid `gvar' tables may keep calling into `ft_var_load_gvar' from
`tt_set_mm_blend' and failing, but the font was invalid anyway and
we want to keep seeing the failure in `tt_set_mm_blend'.

* src/truetype/ttgxvar.c (ft_var_load_gvar): Calculate length of
offset array once.
Allocate arrays after `FT_FRAME_ENTER' (extra check before
allocating and avoid needing to free array later if error entering
Always call `FT_FRAME_EXIT'.
Consistently set counts immediately after array initialized.
Reset the blend (particularly `blend->glyphoffsets') on failure.
2 files changed