tools lib traceevent: test correct variable after allocation

we've tested the wrong variable for allocation failure, fix it to
test the right one.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1356120062-2648-1-git-send-email-sasha.levin@oracle.com
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/traceevent/event-parse.c

@@ -2481,7 +2481,7 @@ process_dynamic_array(struct event_format *event, struct print_arg *arg, char **
 
 	free_token(token);
 	arg = alloc_arg();
-	if (!field) {
+	if (!arg) {
 		do_warning("%s: not enough memory!", __func__);
 		*tok = NULL;
 		return EVENT_ERROR;