[psaux] Full bounds check for OtherSubr 19.

It is possible for OtherSubr 19 to be invoked when `decoder->buildchar` is
NULL (so that `decoder->len_buildchar` is 0), the `blend` is non-NULL with
`blend->num_designs` set to 2, and the user supplied `idx` to be large (for
example 0xFFFFFFFE).  Since these are all `FT_UInt32` the existing bounds
check overflows in a well defined manner, allowing for an invalid call to

In addition, it is possible to call OtherSubr 19 with
`decoder->len_buildchar`, `blend->num_designs`, and `idx` all zero (implying
that `blend->weight_vector` and `decoder->buildchar` are NULL).  This passes
the bounds check (it is logically always fine to copy nothing starting at
index zero) but may invoke undefined behavior in `ft_memcpy` if it is backed
by `memcpy`.  Calling `memcpy` with either the `src` or `dst` NULL is
undefined behavior (even if `count` is zero).

* src/psaux/psintrp.c (cf2_interpT2CharString): Correctly check that
`blend->num_designs` can be copied to `decoder->buildchar[idx]`.
Also avoid passing NULL to `ft_memcpy`.

Bug: https://crbug.com/1299259
1 file changed