We explicitly call Driver::unload, but so does the dtor. We just needed
to clear the dll member after unloading.

Bug: b/177624844
Change-Id: If587df197c2e6242892e41e715885a92167730a7
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52752
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>