vk-unittest: fix crash when unloading shared lib twice

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>
diff --git a/tests/VulkanUnitTests/Driver.cpp b/tests/VulkanUnitTests/Driver.cpp
index d44a240..a3a5c5b 100644
--- a/tests/VulkanUnitTests/Driver.cpp
+++ b/tests/VulkanUnitTests/Driver.cpp
@@ -153,6 +153,8 @@
 	dlclose(dll);
 #endif
 
+	dll = nullptr;
+
 #define VK_GLOBAL(N, R, ...) N = nullptr
 #include "VkGlobalFuncs.hpp"
 #undef VK_GLOBAL