vulkan: Don't use pointer to dropped PhysicalDeviceDriverProperties.

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.

wgpu-hal/src/vulkan/adapter.rs

@@ -907,7 +907,7 @@ impl super::Instance {
                 _ => wgt::DeviceType::Other,
             },
             driver: unsafe {
-                let driver_name = if let Some(driver) = phd_capabilities.driver {
+                let driver_name = if let Some(ref driver) = phd_capabilities.driver {
                     CStr::from_ptr(driver.driver_name.as_ptr()).to_str().ok()
                 } else {
                     None
@@ -916,7 +916,7 @@ impl super::Instance {
                 driver_name.unwrap_or("?").to_owned()
             },
             driver_info: unsafe {
-                let driver_info = if let Some(driver) = phd_capabilities.driver {
+                let driver_info = if let Some(ref driver) = phd_capabilities.driver {
                     CStr::from_ptr(driver.driver_info.as_ptr()).to_str().ok()
                 } else {
                     None