trace-cmd listen: Add NULL pointer to end of cpu_fifos array

add_domain() walked the list of cpu_fifos array that was created by
tracecmd_msg_get_connect(), expecting it to end with a NULL pointer. But it
was not. Luckily valgrind pointed out this mistake.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>