perf evsel: Avoid SEGV if delete is called on NULL

Seen in "perf stat --bpf-counters --for-each-cgroup test" running in a
container:

  libbpf: Failed to bump RLIMIT_MEMLOCK (err = -1), you might need to do it explicitly!
  libbpf: Error in bpf_object__probe_loading():Operation not permitted(1). Couldn't load trivial BPF program. Make sure your kernel supports BPF (CONFIG_BPF_SYSCALL=y) and/or that RLIMIT_MEMLOCK is set to big enough value.
  libbpf: failed to load object 'bperf_cgroup_bpf'
  libbpf: failed to load BPF skeleton 'bperf_cgroup_bpf': -1
  Failed to load cgroup skeleton

    #0 0x55f28a650981 in list_empty tools/include/linux/list.h:189
    #1 0x55f28a6593b4 in evsel__exit util/evsel.c:1518
    #2 0x55f28a6596af in evsel__delete util/evsel.c:1544
    #3 0x55f28a89d166 in bperf_cgrp__destroy util/bpf_counter_cgroup.c:283
    #4 0x55f28a899e9a in bpf_counter__destroy util/bpf_counter.c:816
    #5 0x55f28a659455 in evsel__exit util/evsel.c:1520
    #6 0x55f28a6596af in evsel__delete util/evsel.c:1544
    #7 0x55f28a640d4d in evlist__purge util/evlist.c:148
    #8 0x55f28a640ea6 in evlist__delete util/evlist.c:169
    #9 0x55f28a4efbf2 in cmd_stat tools/perf/builtin-stat.c:2598
    #10 0x55f28a6050c2 in run_builtin tools/perf/perf.c:330
    #11 0x55f28a605633 in handle_internal_command tools/perf/perf.c:384
    #12 0x55f28a6059fb in run_argv tools/perf/perf.c:428
    #13 0x55f28a6061d3 in main tools/perf/perf.c:562
Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Florian Fischer <florian.fischer@muhq.space>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230410205659.3131608-1-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/evsel.c

@@ -1541,6 +1541,9 @@ void evsel__exit(struct evsel *evsel)
 
 void evsel__delete(struct evsel *evsel)
 {
+	if (!evsel)
+		return;
+
 	evsel__exit(evsel);
 	free(evsel);
 }