[truetype] Reject 'fvar' loading when `num_instances` is too small.

* src/truetype/ttgxvar.c (TT_Get_MM_Var): Reject retrieving master when
'fvar' values locally do not match with sanitized values from initialization
at `sfnt_init_face` time.

Reported as

  https://bugs.chromium.org/p/chromium/issues/detail?id=1360295
1 file changed