perf script: Fix allocation of evsel->priv related to per-event dump files

perf script: Fix allocation of evsel->priv related to per-event dump files

When printing output we may want to generate per event files, where the
--per-event-dump option should be used, creating perf.data.EVENT.dump
files instead of printing to stdout.

The callback thar processes event thus expects that evsel->priv->fp
should point to either the per-event FILE descriptor or to stdout.

The a3af66f51bd0bca7 ("perf script: Fix crash because of missing
evsel->priv") changeset fixed a case where evsel->priv wasn't setup,
thus set to NULL, causing a segfault when trying to access
evsel->priv->fp.

But it did it for the non --per-event-dump case by allocating a 'struct
perf_evsel_script' just to set its ->fp to stdout.

Since evsel->priv is only freed when --per-event-dump is used, we ended
up with a memory leak, detected using ASAN.

Fix it by using the same method as perf_script__setup_per_event_dump(),
and reuse that static 'struct perf_evsel_script'.

Also check if evsel_script__new() failed.

Fixes: a3af66f51bd0bca7 ("perf script: Fix crash because of missing evsel->priv")
Reported-by: Ian Rogers <irogers@google.com>
Tested-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Link: https://lore.kernel.org/lkml/ZH+F0wGAWV14zvMP@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>