tools lib traceevent: Fix kbuffer_read_at_offset()

When it's called with an offset less than or equal to the first event,
it'll return a garbage value since the data is not initialized.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20161001101700.29146-1-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/traceevent/kbuffer-parse.c

@@ -622,6 +622,7 @@ void *kbuffer_read_at_offset(struct kbuffer *kbuf, int offset,
 
 	/* Reset the buffer */
 	kbuffer_load_subbuffer(kbuf, kbuf->subbuffer);
+	data = kbuffer_read_event(kbuf, ts);
 
 	while (kbuf->curr < offset) {
 		data = kbuffer_next_event(kbuf, ts);