libperf evlist: Fix a refcount leak

Memory leaks found by applying LLVM's libfuzzer on the tools/perf
parse_events function.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: clang-built-linux@googlegroups.com
Link: http://lore.kernel.org/lkml/20200319023101.82458-2-irogers@google.com


[ Did a minor adjustment due to some other previous patch having already set evlist->all_cpus to NULL at perf_evlist__exit() ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>