parse_events() is often called with parse_events_error set to NULL.
Make parse_events_error__handle() not segfault in that case.

A subsequent patch changes to avoid passing NULL in the first place.

Fixes: 43eb05d0 ("perf tests: Support 'Track with sched_switch' test for hybrid")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20220809080702.6921-2-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>