d3d12: Fix path passed to CreateDeviceFactory

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27287>

src/gallium/drivers/d3d12/d3d12_screen.cpp

@@ -1382,6 +1382,7 @@ try_find_d3d12core_next_to_self(char *path, size_t path_arr_size)
       return nullptr;
    }
 
+   *(last_slash + 1) = '\0';
    return path;
 }
 #endif