CVE-2020-36434: Double free in sys-info

Affected versions of sys-info use a static, global, list to store temporary disk information while running. The function that cleans up this list, DFCleanup, assumes a single threaded environment and will try to free the same memory twice in a multithreaded environment. This results in consistent double-frees and segfaults when calling sys_info::disk_info from multiple threads at once. The issue was fixed by moving the global variable into a local scope.