Avoid gcc warning.

Introduce a default case for the body of the switch statement in
print_statistics() to avoid the following gcc-4.8.1 warning:

	select.c: In function 'print_statistics':
	select.c:806:2: warning: enumeration value 'select_mode__NULL' not handled in switch [-Wswitch]
	  switch (select_conf.select_mode_arg) {
	  ^