tracing/filters: fix NULL pointer dereference

Try this, and you'll see NULL pointer dereference bug:

  # echo -n 'parent_comm ==' > sched/sched_process_fork/filter

Because we passed NULL ptr to simple_strtoull().
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Tom Zanussi <tzanussi@gmail.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <49E04C43.1050504@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace_events_filter.c

@@ -410,6 +410,11 @@ int filter_parse(char **pbuf, struct filter_pred *pred)
 		}
 	}
 
+	if (!val_str) {
+		pred->field_name = NULL;
+		return -EINVAL;
+	}
+
 	pred->field_name = kstrdup(pred->field_name, GFP_KERNEL);
 	if (!pred->field_name)
 		return -ENOMEM;