perf tools: Use zfree() instead of ad hoc equivalent

We have zfree(&ptr) for this very common pattern:

   free(ptr);
   ptr = NULL;

So use it in a few more places.

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1485952447-7013-4-git-send-email-treeze.taeung@gmail.com


[ rewrote commit log ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>