python: Fix convert_tuple_to_char_pointer_array

This finally fixes a few issues with the magic
convert_tuple_to_char_pointer_array function.

This now clearly copies the char* from the python object so we don't
end up keeping reference to those.

Also add the few required free calls to free the content of the array.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>