`FT_NEW_ARRAY(p, 0)` sets `p` to `NULL`. `FT_Stream_ReadAt` with a
memory based stream uses `FT_MEM_COPY` which is `memcpy` which specifies
that it is undefined behavior for either the `src` or `dst` to be
`NULL`. Instead of forcing all callers work around calling
`FT_Stream_Read` when `buffer == NULL && count == 0` do the check in
`FT_StreamRead`. This allows any call with `count == 0` to succesfully
read zero bytes without UB.
* src/base/ftstream.c (FT_Stream_ReadAt): skip `FT_MEM_COPY` when
`count == 0`. (FT_Stream_TryRead): ditto
Fixes: #1250
a9793feace