perf tools: Avoid double free in read_event_file()

The temporary 'buf' buffer allocated in read_event_file() may be freed
twice.  Move the free() call to the common function exit point.

Fixes the following coverity complaints:

  Error: USE_AFTER_FREE (CWE-825):
  tools/perf/util/trace-event-read.c:309: double_free: Calling "free"
  frees pointer "buf" which has already been freed.

Signed-off-by: Sanskriti Sharma <sansharm@redhat.com>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Cc: Joe Lawrence <joe.lawrence@redhat.com>
Link: http://lkml.kernel.org/r/1538490554-8161-5-git-send-email-sansharm@redhat.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>