radv: Fix robustness violations of out-of-bounds index buffer reads

These patches address relate to some edge cases in our robustBufferAccess2 implementations. In particular some games call vkCmdDrawIndexed with firstIndex+indexCount pointing past the end of the index buffer, hence requiring us to either omit those calls or clamp them properly.

In addition to that, 5eedaa9b is arguably "in spirit" of robustness and seems like the kind of thing DXVK titles might run into, but I'm not 100% sure we really need it.