vulkan: Fix assignment of vkGetInstanceProcAddr on Windows.

"*(void**)pfn = LoadAddress()" would cast the NULL pointer in pfn to a
void**, and then dereference it, which wasn't what we wanted. Replaced with
a clearer cast operation.

src/video/windows/SDL_windowsvulkan.c

@@ -57,7 +57,7 @@ int WIN_Vulkan_LoadLibrary(_THIS, const char *path)
         return -1;
     SDL_strlcpy(_this->vulkan_config.loader_path, path,
                 SDL_arraysize(_this->vulkan_config.loader_path));
-    *(void**)vkGetInstanceProcAddr = SDL_LoadFunction(
+    vkGetInstanceProcAddr = (PFN_vkGetInstanceProcAddr) SDL_LoadFunction(
         _this->vulkan_config.loader_handle, "vkGetInstanceProcAddr");
     if(!vkGetInstanceProcAddr)
         goto fail;