If two threads would call sf_free() / free_memory() at the same time,
bad_ptr() would not detect this. Fixed by adding extra detection
when working with the memory region under sf_mutex.

Other things:
- If safe_malloc crashes while mutex is hold, stack trace printing will
  hang because we malloc is called by my_open(), which is used by stack
  trace printing code. Fixed by adding MY_NO_REGISTER flag to my_open,
  which will disable the malloc() call to remmeber the file name.