trace-cmd: Fix a crash on "trace-cmd reset" command

A leftover in reset_cpu_mask() function causes a segmentation
fault of "trace-cmd reset".

Link: http://lore.kernel.org/linux-trace-devel/20191018145759.15335-1-tz.stoyanov@gmail.com

Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>