In the past this wasn't needed as the libaudit based code would use just
one field, and the alternative constructor would fill in all the fields,
but now that even when using the libaudit based method we need the other
fields, switch to zalloc() to make sure the other fields are zeroed at
instantiation time.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>