Add finalization safety check to com_err

Add finalization safety check to com_err

If the linker erroneously runs the libkrb5 finalizer after the
libcom_err finalizer, the consequent remove_error_table() calls could
crash due to accessing a destroyed mutex or an invalid et_list
pointer.  Add an unsynchronized check on finalized in
remove_error_table(), and set et_list to null in com_err_terminate()
after destroying the list.

[ghudson@mit.edu: minimized code hanges; rewrote comment and commit
message]

ticket: 8890 (new)