perf parse-events: Check if the software events array slots are populated

To avoid a NULL pointer dereference when the kernel supports the new
feature but the tooling still hasn't an entry for it.

This happened with the recently added PERF_COUNT_SW_CGROUP_SWITCHES
software event.

Reported-by: Thomas Richter <tmricht@linux.ibm.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>
Link: https://lore.kernel.org/linux-perf-users/YKVESEKRjKtILhog@kernel.org/


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