zink: fix slot-map checks

zink_gfx_program::shader_slot_map is an array of unsigned chars, so comparing against negative values isn't going to work. Instead, we need to compare against 0xff, which is what we use elsewhere.

While we're at it, clean up the memset to make it a bit more clear.

Fixes: 5c5e1abe ("zink: evaluate existing slot map during program init and force new map as needed")