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 <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Sasha Levin <[e-mail]>
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/lib/traceevent/event-parse.c

···
 
 	free_token(token);
 	arg = alloc_arg();
-	if (!field) {
+	if (!arg) {
 		do_warning("%s: not enough memory!", __func__);
 		*tok = NULL;
 		return EVENT_ERROR;