libperf: Add NULL pointer check for cpu_map iteration and NULL assignment for all_cpus.

A NULL pointer may be passed to perf_cpu_map__cpu and then cause a
crash, such as the one commit cb71f7d4 ("libperf: Setup initial
evlist::all_cpus value") fix.

Signed-off-by: He Zhe <zhe.he@windriver.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Kyle Meyer <meyerk@hpe.com>
Link: http://lore.kernel.org/lkml/1583665157-349023-1-git-send-email-zhe.he@windriver.com


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