perf evsel: Initialize evsel->per_pkg_mask to NULL in evsel__init()

perf evsel: Initialize evsel->per_pkg_mask to NULL in evsel__init()

Just like with the other fields, this probably isn't fixing anything
observable as evsel__new() uses zalloc() for the whole 'struct evsel',
but since evsels can be embedded in larger structures and maybe those
larger structures don't use zalloc() for some reason, init it to NULL
just in case.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
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>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>