Detected by ASAN, but seems impossible to exploit.

Since `ash::vk::definitions::PhysicalDeviceDriverProperties` is
`Copy`, the statement

    if let Some(driver) = phd_capabilities.driver { ... }

actually makes `driver` a local copy of the struct. The code uses
`as_ptr` to create a pointer to the `driver_name` field of this local
copy, and then tries to use that pointer outside the `if let`, when
the local copy has gone out of scope. This is UB.

Taking a reference to the properties struct is correct and more
efficient.

Remove a handful of `unsafe` blocks from the Vulkan back end.