perf session: Fix crash with invalid CPU list

commit 5d67be97 added the option to specify a range of CPUs of interest,
but does not catch an invalid CPU list:

$ perf script -c foo
Segmentation fault (core dumped)

Cc: Anton Blanchard <anton@samba.org>
Link: http://lkml.kernel.org/r/1321206327-5881-1-git-send-email-dsahern@gmail.com


Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>